There already exist commercial clones of the 4004, but w.r.t. DIY the easiest way to get to the guts would be through the Verilog version on an FPGA [0].<p>Alternatively you could go full monty and build something like this <a href="http:&#x2F;&#x2F;www.megaprocessor.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.megaprocessor.com&#x2F;</a><p>[0] <a href="http:&#x2F;&#x2F;opencores.org&#x2F;project,mcs-4" rel="nofollow">http:&#x2F;&#x2F;opencores.org&#x2F;project,mcs-4</a>