It’s a bit confusing! Apple only tested 4 GB and only supports 4 GB.
But, things are not so bleak! You can go with 6 GB and you might be able to go with 8 GB! The issue is the quality of the RAM. The RAS and CAS of the given SO-DIMM may not work well with the slower RAM I/O channel this system has, you want slower SO-DIMMs not faster (1066 MHz PC3-8500 DDR3 SDRAM)
You also need macOS Snow Leopard (10.6.8) or newer upto El Capitan 10.11.6 with Boot ROM Version MB51.007D.B03. Sierra (10.12.6) and newer is not workable as the firmware is not 64-bit clean.
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crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
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