There is a long thread about 4GB in the Si 1520 in the german amilo-forum:
https://www.amilo-forum.de/topic,21781,-si-1520-mit-4GB-Ram.htmlThere have been numerous tries and errors and the essential conclusion is, if you run BIOS 1.10 4GB are likely to work but not for sure.
Some report that they see 3.3GB in the POST. I can confirm this from an earlier test with BIOS 1.20, but neither Win XP nor openSuse 10.3 nor Debian Etch (all 32bit) were able to load the graphics drivers with 4GB installed. The system itself ran fine, but the screen turned black when the graphics drivers were loaded.
Recenty I tried it once again with Debian Lenny (64bit) but even BIOS 1.10 didn't detect more than 3GB in the POST if 4GB are installed. The OS ran fine (with graphics) but since the BIOS did only report 3GB, the OS couldn't use more.
In my case the RAM modules have all been of type PC2-5300 by MDT. But since I gave away my second 2GB module after the first test, they were not the same for the second test. Although the specifications were identical. Others reported, that they had success with the same modules like mine. Others again reported, that they could make it work with Kingston modules.