Z80A 4MHz NMOS microprocessor made by many brands. This is used in many arcade games and early home computers. All have tested to at least 4MHz. These are the most common speed in the very old arcade games.
Datasheet
Zilog Z08400x NMOS Speed Chart:
Z084002= Z80 at 2MHz
Z084004= Z80A at 4MHz
Z084006= Z80B at 6.17MHz
Z084008= Z80H at 8MHz
These have all been tested and verified NMOS in a Z80 Tester. See images below. These came from working arcade game boards.
