A friend was telling me about Microsquirt, is that a viable option for a 4 cylinder Miata? What are the downsides compared to Megasquirt?

 

everyone with a DIYPNP runs microsquirt.

 

Downside is that it requires wiring in.

It is what I will be using on my full race miata buildwhere iI'm rewiring the whole car.

 

Does a microsquirt come with a base map installed?

 

No, you have to load it just like a PNP MS. Functionality is close enough that I've never given any though to the fact I'm running a Microsquirt module.

 

Microsquirt is a MS2 in an easy to package SMD board.

If you bought just the uS board there will just be the default .msq on it, no basemap.

 

Essentially true. The Microsquirt EMS uses the same processor as MS2, but to save space (for packaging), Microsquirt does not support low impedance injectors or stepper idle control. I don't see low impedance injectors used commonly in Miatas, and Miatas don't have stepper idle valves, so this is probably not a big deal. However Microsquirt V3 does use the MAX9926 "smart" signal conditioner on both crank and cam inputs, just like MS3-Pro. Some would consider that a big step up.
Otherwise, Microsquirt is pretty much a smaller, lighter, better sealed version of MS2.