When using Rules to set a value on a taxonomy term reference field, I get choices like "field:0", "field:1", etc. What do these numbers mean?
It's basically letting you have access to a zero-based array for the different instances of a multiple cardinality field (as I think you've already worked out).

It seems the reason there are 4 shown is just a default; a visual cue to let you know there are more than one possible values for that field. 

If you have a field with 20 values you'll still only see 0...3 in the Rules data selector, even though you can use 0...19.