This is not true. The AI works with many hidden parameters and if your actions resemble any remotely real situation you will note that more doors open. If you play in a way that attempts to game the game you will see some close but you will obviously brutally overpower the AI for a period of time.
As you may have noticed trade embargos are hidden in Civ 4. But if you attempt to game the game and trade with everyone like most "pro players" here do (among other gamey things like shutting off research at the eve of the final war - I do game the game a bit to!) then the AI will in the end cease trading with you completely because most have a trade embargo against someone and if you trade with everyone then nobody will trade with you.
This does not go for friendly civilizations though, at least not most of them time. It is usually over-written. But like one of the few sensible players here pointed out and later in PM guided me to an excellent guide dealing with AI
battle mechanics, even this can (probably) be overwritten once a civilization deems you to be the only threat to progress or victory.
Don't quote me on the last one though. I'm not 100% sure.
Traius, Hope I can help a lil bit at least
To play the game this more lame way you should like poster above said go for an early unique tech path like aesthetics (in old civ 3 it used to be republic). But it is certainly possible to push yourself through conventional techs as well, picking a strong, longterm path and then trading for several surrounding techs.
The AI does NOT increase the cost of techs in any way against you as time passes on purely on the basis that time has passed.