Looks like you were looking for the ARM4 and ARM5 microarchitectures. During the time when ARM Ltd. was spun-off from Acorn Computers in 1990, the ARM4 and the ARM5 were skipped for some reason. Those were never commercially announced nor produced. It's unknown if they were designed internally and were scrapped or if they were simply renamed for some particular reason.

• 1989: Acorn's ARM3 introduced
• 1990: Advanced RISC Machines, Ltd is established
• 1993: ARM's ARM6 introduced