[lsiebert]

you need two zero cost creatures and a way to sacrifice them for benefits, then just trade off. I can think of infinite mana with four cards.

[JoshTriplett]

Magic is not a game that considers all combos a bug, and there are any number of ways to get to infinite mana (or infinite many other things) with less than four cards.

[jldugger]

Yep, Ashnod's Altar, that card, and any 1-2 cost artifact creatures and you're set.

FWIW, it's about the same cost as Enduring Renewal.