4x4 oak is available dried, it is just a matter of being careful that it has been dried properly.
You could try EC Forrest 01825 872025. I think they are trade but will probably sell a piece or two if you collect it. They are on the A22 North of Eastbourne. Don't know how far that is from you. I've never actually used any from them but they have offered me 4" Kiln dried squares in the past. Probably wont be cheap though.
I usually glue two 2x4's together, but that's because we are using american white oak at the moment.
If you do glue up the best glue I have found for the oak is Bison Timber Max. Get very few problem with glue lines opening up with that one. If you glue 2x4's sand the faces lightly with a coarse grade paper across the grain, helps the glue soak into the timber, and completely cover both faces with a very thin layer of glue. This gives a very reliable joint.
Johnny B