Dell will ship most of their servers, including
the 1950s and 2950s, with RH4.  So I would expect
SL4 to work fine.  We're just getting ready to
use SL4, but we have run RH8, RH9 and SL3 on a number
of Dells with no problem.  We always bought them
with no OS and installed something we had.  We just
made sure we used whatever they currently would have
provided or later.