Yep, 4MB max right up to everything but the Classic II, the SE/30 was a Mac_IIcx in sheep's clothing and the CC is anything but a Compact.

The 512 on your MoBo is ignored, or the top end of the four above the floor/whatever. It might be cool to try that board in a Plus if it turns out to be compatible. I'm wondering if CompactVirtual could be persuaded to work in that config, like they used to do for the additional memory slots on some of the later SE accelerators.