Memory defined at boot time incorrect

Sorry if I'm missing something obvious here.

I have a new-ish motherboard with an amd64 capable CPU, however I'm
using i386 on it because this system has existed since long before
amd64 was a thing.

At boot time only about 2 gigs of memory is being discovered, even
though 4 gigs is installed. I see some options in the kernel config to
define the memory size but so far I haven't figured out the right
combination of settings, everything I've tried causes compile errors.

What am I doing wrong?

Thanks.

Andy