SuSE 10.1 32bit and 4GB RAM, seen are 3.5GB

Hi, I'm struggeling with the following: My computer, a Dell Optiplex 620 has a Intel P4 XEON HT with 4x 1GB RAM. The kernel sees 3.5 GB only. By default the kernel has HIGHMEM enabled and is set to 64GB. I recompiled the kernel so that it is compiled for 4GB highmem only. Same result. I enabled the hidden CONFIG_EMBEDDED and selected the 3GB/1GB memory split. Same result. It seems, no matter what I do, all I get are the 3.5GB. The funny thing is, a slackware edition was run before on the same machine, and it detected the 4GB. What could I do? (not too keen about installing slackware....) Thanks! Regards, Martin