Basically Thief 4 was going to be a retelling of the original games, except set in modern times. It would have modern versions of the same characters from the old games, who's roles would have been translated to modern day parallels. Some of the old maps were going to make a reappearance as well. To be clear, it was not going to be anything like Splinter Cell or similar games. Garrett wasn't going to be turned into a James Bond type of guy with a bunch of gadgets or anything like that. He would still be the dark, sarcastic loner who uses his own skills and ingenuity to do his thieving, not gadgets. I don't think we were even going to let the player use a gun, just simple tools and weapons. The art direction would have been similar to the steampunk from the previous games, just translated to modern times, with the same crowded and dilapidated city streets and such. I guess the Silent Hill series is the closest reference I can think of for what we had in mind for looks, but it would have been pretty different from that too. We were going to somewhat modify the "rules" of the modern world to make them more conducive to the Thief-type stealth game also, and not necessarily realistic. There wouldn't be light switches that anyone can just turn on to reveal you, the enemies wouldn't all have machineguns, there wouldn't be lasers, etc. Basically we were going for a very similar type of gameplay to the original series, just in a twisted steampunkish modern setting.
It was only in pre-production for about two months before the Ion Storm "end times" began, so the concept never got much more fleshed out than this, but that's basically what it would have been.
It would have used the same engine as T3, but we were hoping it would be optimized a bit more and get some new features.
