I target my batch sizes at 4.75 gallons, but usually end up closer to 4.5 (I think - I really need to make a dipstick to measure it). I use Fermcap to keep krausen from coming up too high, but sometimes I still get some foam in the blow-off tube, which I just connect using a regular old gas QD. I've never had it clog.
If you're really concerned, you could drill a large enough hole in the lid to connect a proper blow-off tube, but I really don't think you'll find that necessary. An other option is to take off the gas post entirely and stretch a section of tubing over the threads.
I've never brewed a lager, so I can't help you there.