The minimum wage in the state in question is actually $7.25/hour. However the restaurant can pay servers as low as $2.13/hour directly, but have to make up the difference if their tips do not bring them to the $7.25 rate. My guess is that Sonic has calculated that $4.25 is the proper base wage for that particular role. There may even be some sort of tip pooling going on for people working the windows.

Fact is the federal minimum wage is $7.25. The "make-up" rule applies in any state where a server is paid less than the benchmark.

http://www.dol.gov/elaws/faq/esa/flsa/002.htm