The V4 units are hackable, but they are very tempermental. You can't flash them initially with a firmware that is larger than 3 Mb and sometimes they get messed up and/or stuck and end up bricks.
Anyway, the best thing to do with a V4 is to first go to
www.dd-wrt.org and download one of the v23 beta firmwares, try the latest which right now is from 11/04/05. (Since they are not located in the US they use 04-11-05 for the date.) Anyway, they have a mini, std, and voip sizes. You need to flash the mini first, since it's under 3 MB. After that you can put the std one on. Oh, and make sure to use the Generic .bin files, not the one for the G. I know it sounds strange, but trust me.
Or if you don't like dd-wrt you could use some firmware from Tofu or Thibor. Tofu is for the WRT54G and Thibor's builds are for the GS.
The V4 G is pretty cool though, the hardware is the same as the V4 GS so you can put Thibor's GS firmware on it and it will be a GS with speedbooster. (Of course, you can get that from dd-wrt too. It's a checkbox you turn on in the GUI.)
This is because Linksys has cheapened the GS as well as the G now and the V4 GS and the V4 G are the same, 4 MB of flash memory, and 16 MB of other memory. The older GS had 8/16.
Get even more info from www. linksysinfo.com and
www.dd-wrt.com
Just stay away from the V5 G which is 2/8. Nobody can do anything with them yet and probably never will.