why pae kernel has only 3G memory

[lou]

i have stretch, it use 3G memory, 1G memory can't be used

top: KiB Mem :  3038696 total,  2221904 free,   400628 used, 416164 
buff/cache
uname: Linux debian 4.9.0-13-686-pae #1 SMP Debian 4.9.228-1 
(2020-07-05) i686 GNU/Linux

i think pae kernel can use 4G memory

do i have to upgrade to buster or bullseye?