Ok, I guess you two aren't engineers, so you don't know about this stuff. No user is going to port this game to the PC. You are right that the developement architecture is very similar (as they can both use mostly the same codebase, and use the same development environment). However, the processing architecture is COMPLETELY DIFFERENT (RISC, PPC, big-endian, and in-order-execution vs CISC, x86, little-endian, and out of order execution). When you compile an application to distribute, it is torn apart and reassembled into something almost nobody can make any sense of. There is a compiler for the PC, and a compiler for the xbox, and the outputs are so different that nobody in their right mind would ever try to make a direct port. Compiled code looses all formatting and organization, making it unintelligible. The job becomes even more difficult because the xbox has a completely different set of processor instructions compared to x86 PC's. Once you end up with that compiled results, the xbox is about as much like a PC as a steam engine is like an ion drive. Decompiling is the only way significant changes could be made, and it s nearly impossible to use to read PC games which were built with open-source compilers! You still loose all the formatting and structure, and there are MANY common errors. However, even the terribly frustrating process of decompiling is not going to happen for the 360 any time soon, because crackers don't have the 360 compilers (which are held secretly by Microsoft) to begin with! With no list of instruction functions, or a compiler to test things on, decompiling becomes essentially impossible (at least on this massive scale).
Someone will bring up emulators in a few posts, but right now there is no 360 emulator that works for ANY commercial game, and emulation of a game like GTA4 (which would require a top-of-the-line PC to play even if R* optimized it) would run at about 10 seconds per frame on the most powerful PC's. I mean, there is an emulator out there for the original xbox which could play 1 game (Halo 1), although at an unplayable performance with MANY MANY MANY crashes, and very very limited video card support. If it does come this way, it will not be playable for at least 5 years (and that is very, very generous).
Additionally, we are at the point where the 360 is pretty up-to-date with graphics cards. It uses a certain bit of the DX10 specification, meaning that it is only going to be playable on the Radeon 2800+, and MAYBE 8800+, unless someone rewrites the entire rendering pipeline (yes, that means rebuild all the effects which make it pretty) to support lower hardware.
Even worse, the entire memory management system on a 360 is different than on a PC (one RAM pool), meaning someone else will have to chuck in lock() and unlock() operations (which is difficult, and it will greatly slow it down as the rendering pipeline cannot be restructured given the difficulty of dealing with decompiled code).
So, what you mean to say was; "no, GTA4 is not going to move to the PC from a cracked 360 version because it is more likely that George Washington will rise from the dead to battle flying pigs and unicorns."