Welcome to the best game ever!
First, it would help to know what settings you are currently playing, as certain settings can make the game even more challenging for newer players. I'll assume you are playing BTS.
There's been discussions in the past of the major difference between IV and V, but I can assure you otherwise that they are very different games. You can google for that info for sure, but I'll provide some highlights.
1) Happiness is local. Global happiness was a terrible mechanic
2) Your main limiter to expansion is maintenance, and there are different types. You will get more savvy at understanding this as you play the game more. But distance to cap maintenance is was one of the big ones to understand for now. Try not to overextend your empire early and settle too far away. Don't be afraid of city overlap, which is actually a good thing in IV. Allows tile/resource sharing which is nice early when happiness is low anyway, as well as keeps distance maintenance lower. (Maintenance increases as you move up levels you should note)
3) Workers are for more important in IV. In fact, it is recommended that you almost always build a worker first. There are a exceptions, but rarely..mostly to do with certain coastal starts and starting techs/resources visible. But for now, worker first is a very solid play
4) Scouts on the other hand are not important..at all. Unless you start with one due to Hunting starting tech, you will not build them. In V, common play is to build a couple of scouts straight up for ruins and meeting AI. Well, many here don't play with huts on anyway, but once you play a certain difficulty grabbing huts is not a big deal at all, and AIs start with a couple themselves, so it is really a waste.
5) Further point to 4, barbs are far more dangerous in IV, so really your objective here is to use initial warrior or scout to scout the immediate area - apprx 10 tile radius - for city spots and then use them to spawnbust the area. What I mean by spawnbust is barb spawn prevention. A single unit will bust a 5X5 area from the tile it stands on..meaning no barbs can spawn in that area. This becomes more important as you move up levels and barbs spawn more aggressive. But it is a good practice to start now. Scouting is important, but not as crucial early like in V as there is not much interactions until after writing and later Alphabet and Currency. Ofc, they will meet you as well unless you are are certain map types.
6) Worker management is also very important. Use workers wisely. Improve food specials first then get up some production improvements, then chop forests
7) Teching in V is rather cookie cutter. There's a bit more strategy and variability to teching in IV. But the important things to note here are that you can a) Trade Techs after alpha b) AIs prioritize certain techs c) Early religious techs are not the way to go nor early religion. So the key here is to focus on worker techs immediately, especially food. Analyze your starting resources and you starting techs, and make sure you go for any food techs you need first. Then strat techs like Bronze Working and Animal Husbandry (also food tech) for copper and horse. Most often teching Iron Working is not a good idea at all. It is expensive early and an AI priority tech that you can easily pickup later after Alpha. Next, The Wheel, if you don't start with it, to hook up resources, if needed, and connect the all important trade routes... then Pottery>Writing>Alpha>Currency is usually good play on lower levels. After Alpha you can easily backfill all the other early techs.
8) Big one here - Slavery is really the most powerful thing in the game and something you should get a better understanding about. It's far more complicated than it initially appears on the surface as well. But I'll just say it is the main reason you hear folks say things like "FOOD IS KING" in Civ IV. Food IS Production. We can go over this more later if you actually run a shadow game or whatever.
9) Research - One of the big differences between IV and V is that research is far more complicated than the simple "Population = Science". Your primary source generally is Commerce, which is the gold looking coins on the map. Those coins are not gold. Gold has a different icon. Commerce is used for beakers, gold, culture and espionage (i assume you are playing BTS), and is altered by the sliders at the top left. Generally, first thing, you will be running 100% science for the early game, but once you settle your first city ..boom..you start running a deficit. At some point soon you will have to start playing with this slider to fund expansion and research. I won't go into detail now, as it is one of the more complex aspects of the game. But note that you should not be afraid to run 100% tax (or 0% research) for some turns to fund you next tech. This is often done once Writing arrives.
10) Great People are very important, and you can start generating them far earlier in IV. Generally, you want to get a Library up fast in your cap and run 2 scientists there. I usually use the first GS for an academy in the cap which is generally your main research hub, which is often setup for Bureaucracy civic. That will really help with the early, and later, research. There are other advanced plays for the early GS or even other great people, but no need for that now.
11) Cottages...Trading post are the closest counter-part to this improvement in V, but trading post are rather static for the game. Cottages grow and get better over time, as you may have realized already. They only grow when worked by a citizen, so try to keep them active as much as possible as these are a main source of commerce. But you don't need them everywhere. This is where city specialization, another important aspect of IV, comes into play. Try to setup cities to specialize as commerce cities. These are usually cities with lots of green river tiles or flood plains. Ideally, your capital will be one of these cites, but not always (you can move the Palace if needed).
12) Lastly, for now, I'll mention that Trade Routes are another important source of commerce. Try to make sure your cities are connected by a road or river initially (ideally the road is in place before you settle as the trade route is instant that turn). Soon after try to get foreign trade routes active as they are more lucrative. On certain maps, the Great Lighthouse is one of the most powerful wonders in the game.
I've requested a move of this thread to Strategy & Tips forum, which is a better source for assistance. I highly encourage that you start a shadow game over there for turn by turn advice and discussion. Great way to learn the game. Early game is so important in IV for the snowball effect. Moreso than V. It can be much harder to catch up in IV if you do a lot of things poorly in the early game.
Oh..and highly recommend the BUG/BULL and or BAT mod for UI enhancements
Not sure what you mean by the diplo screens. I think there is more info available to you in IV. BUG helps with that too. And Diplo is quite a bit more transparent in IV.
edit: Oh..and I highly recommend you play normal settings and maps for now. Pangaea is probably the best map to learn on as you get involved with all aspects of the game early on. Ha. I see a lot of new players come here struggling only to find out they are playing with really wonky settings like normal speed on Huge maps. Normal settings provide a very satisfying game.
