Would you pay 50 bucks if civ 4 was redone to look like civ 5. If the graphics were revamped? I would.
I wouldn't pay $50, but I might pay $20. Graphics aren't as important to me, and in some ways I view the Civ 5 graphics as a mix of improvements and flaws. Civ 5 has higher res graphics, and the leaders look nice, but that's about it. In most other respects, I actually find Civ 4's graphics to be better, especially because of the degree of animation and visual information that conveys. The world feels more alive to me in Civ 4's map.
As for the Steam price, it's currently $30 for Civ 4 complete -- which includes Civ 4 vanilla, and the three expansions (Warlords, Beyond the Sword, and Colonization). I never tried Colonization, and I wasn't too keen on Warlords for the price I paid (I didn't like most of the scenarios, although I did enjoy the extra civs and leaders), but BtS was a lot of fun. For the package deal of $30, it's still a good bargain in my opinion. Worth checking out.
As for Civ 4 vs. Civ 5, plenty has been discussed about their differences. For me, Civ 4 is a lot more engaging. This comes at a price, of course --namely that you can REALLY leverage min/maxing and micromanaging if you care to. I'm talking about managing things like hammer and beaker overflow for production, switching tiles to achieve that, etc. In that sense, from what I can tell, Civ 5 did away with that. Mostly by making everything take FOREVER to do. 1UPT is not my cup of tea, mostly because I've actually PLAYED tactical combat games, and let me tell you, Civ 5 is NOT a tactical combat game. Or at least not a very well designed one. Unfortunately, it's not, in my opinion, a very well designed strategy game, either, because of the sacrifices that needed to be made to accommodate the pseudo-tactical 1UPT change.
To me, all of this makes Civ 5 a game that is built around game mechanics without a real sense of "why" each mechanic exists other than to support other game mechanics. There's less of a unified concept of "why are we designing this stuff this way", and to the extent there is one, it doesn't have to do with simulating, replicating, or even casually representing some general concept about the development of civilizations/nations/empires. Whereas it feels to me that the parts of Civ 4 support an overall holistic concept of the game abstracting how empires develop, Civ 5 feels like a game that involves a bunch of interconnected game mechanics that are designed to support another game mechanic which itself was introduced in response to a PRIOR game mechanic.
To me, it feels as if the designers of Civ 5 said "Man, Stacks of Doom were such a pain in Civ 4. We should make combat a lot more tactical." Then they went about doing that and changing a whole bunch of other things to make their chosen solution (1UPT) functional, without regard to anything else.