memory sticks could be different type or voltage ... boot with one stick, shutdown, boot with the other stick, find out if one is faulty. DELL E5500 uses DDR2-800, non-ecc, unbuffered, 1.8V if that helps.
thanks for the info - I will try that, but in the Bios its being reported as 4G and this is across 2 machines and 4 different sets of RAM. Memory sticks 'match' (pair to pair) so that lessens some of the possibilities.
I haven't tried running memtest. So that's my next step
I'm also going to live boot off 18.04 (10?) {whatever the LTS was] in case it's something peculiar to 19.04.
GNU/Linux makes as much use of the RAM as it can for things like cache, to speed up the system.
So in the free -m command, you'll want to check the "available" column too. The "total" should be around 4096 (4G) if the OS is correctly working with all your RAM, which I suspect it should be if the BIOS reports 4 GiB.
Take a look at this website for more detail on how memory usage works:
After much checking/scratching of head/internet research/running memtest, dmesg, etc, etc I can impart the following weirdness...
On one machine even though bios reported 4G - memtest only reported 2G (as did Ubuntu). After swapping sticks back and forth multiple times I finally managed to obtain 4G by using one stick from each pair. This is despite them ALL being identical Samsung branded 2G sticks.
Have to now try the other machine (with the two remaining sticks) to see what other conclusions I can draw.