Can me and my husband reinfect each other with chlamydia after being treated 4 days ago and had sex last night?

Yes: There is a reason that the test of cure is done at three weeks. However, since both of y'all are treated at the same time it severely reduces the risk of reinfection. I would be more concerned about generating a resistant form of chlamydia. I would suggest following up as you were previously instructed and usually that is three weeks after treatment for the test of cure. If negative at that time you don't have to worry.