Thinking and planning phase.
I want to emulate Intel 4004 (and possibly 4040 too) CPU with full capacity of ROMs and RAMs. Their I/O pins will be limited by the AVR package. Execution should run at the original speed of about 90 kIPS (740 kHz main clock), or possibly faster. Target candidates are ATmega168 or 328 and higher. Board for demonstration could be Arduino Nano or Micro but it can run on naked chip. Assembly only.