I was told this discount applies only in the Portland, OR market, since apparently sales in this market are handled by a different company, or something like that.
The guy telling me this was a LEAP employee working at a Cricket corporate-owned store.
He said that this was applicable anytime and to any account without any limitations. You just need to pay the exact amount for three months, then either go to the store or call Customer Care, and they'll enter in a code that'll give you the fourth month free. After the 4 months, you can do this again. Also, he said you could pay for 6 months of service at a time if you so wanted, and you'd get service for 8 months with this.
I haven't tried it out yet since my bill is already paid for this month (I pay it well ahead of time so their ****ed up billing system doesn't disconnect me).
I'll let you guys know whether or not it works once I try it out.