"I thought there was a way to convert jpegs to .bum files so you could use them in earlier versions."

Poser 4 will convert .jpg to .bum if you load the .jpg manually in the material editor as opposed to using a MAT .pz2 file. You will get poor results however from using a .bum created from a gray scale .jpg intended for P4 Pro Pack and later.

Poser 4 can use .jpg bump maps if you convert them. Here's a link to that explains the process along with a PhotoShop action or instructions on how to do it manually in any image editor that supports channels. You can then use the .jpg's and the MAT files that refer to them and not have the huge .bum files.

www.daz3d.com/i/tutorial/tutorial

Other than the bump map issue, there's no reason why any of the clothing shouldn't work in P4 as long as it's not Dynamic clothing, which will only work in P5 and above. You may well lose any procedural shaders used to change the color, etc. of the piece but it will work otherwise  Poser will ignore the extra stuff it doesn't understand. The same is true for Daz Studio, so the requirements while not completely inaccurate can be a bit misleading. If in doubt, contact the vendor and ask them specifically which features if any won't work.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken