Playhouse South Wall

I am about half way through framing the south wall.  I showed my progress to my neighbor.  He asked me a few questions, which got me second guessing my use of 4x6 blocks as my window and door headers.  The city  did not require any construction details for the building permit, and I guess that's because they consider the playhouse to be just a "utility building."  I just had to tell them how big it will be, where I will put it, and get consent from my neighbors.  So technically my use of 4x6's as headers is legal, but I wonder if they will be strong enough.