SIMM sockets may develop solder fractures or internal breaks that will cause problems, either when the machine warms up or when it tries to boot. Resoldering may help, or it may require replacement of the socket.
The plastic-tabbed SIMM sockets used in the A4000 are fragile, and if the tab breaks off, you'll need to either replace the socket or come up with some other mechanical method to hold the SIMM firmly into the socket.
It's possible to use 8M or even 16M SIMMs for Fast RAM in place of 4M SIMMs, although they may not want to clip into the socket. The memory limit is still 16M of Fast RAM on the motherboard.
A 4M or 8M SIMM may work as a replacement for the 2M Chip RAM SIMM,
although only 2M of Chip RAM will be provided, or possibly only 1M, depending
on the A4000. Later model A4000s have the Chip RAM soldered onto the motherboard.