So, with an 04 you will be OBDII. Like Blackdawg said, you won't be able to do much unless your are running forced induction (is that FAI?) I've seen 4 cylinder turbo tacomas make a lot of power and run aftermarket programs to tune fuel, timing, knock control etc.
If you are set on it, you can check out megasquirt as a fuel management system:
Here's the hardware:
V6 Tacoma stuff
4 cylinder megasquirt thread on customtacos
Some threads of people troubleshooting once they run into problems(tach won't work, truck wont idle, loosing crank signal)
Hope this helps,