1) you don't say whether this is a 386DX (32b memory bus) or a 386SX (16b memory bus), but given the 8 SIMM slots with 2 sets of 4 I will assume it's a DX. In that case memory needs to be installed in 32b blocks, and as each 30p SIMM is 8 bits wide, you have to install RAM per 4 modules. So yes, it's fine to install 4 SIMMs leaving the other 4 empty. Also the two blocks don't need to be identical.

2) depends on the specs. If they are equal to or better than the current SIMMs it will just work. 60ns is capable of higher speeds (up to about 75MHz...) than 70ns (max 60MHz), but that difference is irrelevant on a 386 which will not be faster than 40MHz. Both sets have parity and are FPM and 5V so it will just work. You may be able to tweak some settings for marginally better performance, but given 70ns is already overkill for a 386 I doubt it (unless current settings are needlessly conservative)

Thanks dude. Ordering away! I think the ram amount will make much more difference than anything else. It's nice to make a ramdrive for example to run small games that can fit into it. Or... AutoCAD 🤣.

When I lowered the timings of my 386SX 25 I had to change my RAM to avoid errors : the RAM was 6MB 80ns and it got replaced with 8MB 70ns and that fixed the issues.

And just lowering that one timing made a great performance boost.

Actually, I would first check if the mainboard supports 4 MB SIMMs at all. While it should, you never know for sure until checking.