High five for you! You do not want longer than 4.5". It's even trickier than you can imagine because the tendency to measure ar15 barrels at the receiver face negates the added length created by the dedicated conversion kits. Simply put, the barrel is longer than it should be because no one thinks of the change in relationship between the bolt face and chamber.
I had a huge go round with Spikes Tactical over this. I'm in possession of one of the three they ever made. But it's a complete upper and I'm not to keen about parting it out for you.