The requirements for the 4MB SIMMs are the same as for standard Macintosh
II SIMMs, but they must be rated at 100ns or faster. They must also fit in
a significantly smaller space than is allowed by the Macintosh II family.
This eliminates 4MB SIMMs made from lots of 1MB DRAM chips.
A number of vendors advertise compatible 4MB SIMMs. Just check the ads in
your favorite Macintosh-oriented magazine, or check with an Authorized Apple
Dealer.