I know it's cliche to say so, noypi, but the search engine is your ultra-special pal.
The gentleman who mods 4Gs has stated he doesn't perform surgery on smaller models. In the first place, he considers the DAC in the 4G to be superior to that of the 3G, 5G, Mini or Nano. In the second, the 4G affords him just enough room in which to work (he does the mods himself -- by hand). He has modded at least one 5G successfully as an experiment, but the process was harrowing, interminable, and, he concluded, not worth it. Horror stories from others about changing the 3G's battery seem explanation enough for his reluctance to touch that model.
Hence the recent threads in this forum discussing good deals on the discontinued iPod Photo 60gb. Search for those threads, if you like; the 4G models haven't quite disappeared completely, so you can still find the certain of the bargains noted in those threads. A few months hence, I doubt that will be the case -- especially for two of the best deals mentioned.