Would it be possible....

[Lower-Than-Low]

To put the top four strings from a 5-String bass onto a 4-srting bass?

[KeithBMI]

Yes. You may need a new nut.

[cdef]

Also, a fat B string may not fit through the hole in the bridge for the original E string, in which case you'll need to file it out a little.

[Lower-Than-Low]

Interesting... I find playing with a five string a little bit awkward but I like the really low sound I get from the added string. I'll keep this in mind if I ever get the chance to modify my bass.
Thanks for the help!

[BusterDog]

I did that once. I had to move my bridge back a little to get it to intonate properly.