I would have thought that 4" might be a little on the large size for a 38" boat.
I have a 40' racer crusier and only use 3" with 1/8" wall (approx).
Buy your fittings from a chandlery or on-line (Mauri Pro are
cheap with good service), buy your alloy tube (6000 series is best if possible - I think I went with 6061, but take advice if not sure), but from a metal supplier not a chandlery - it will save you plenty (I just bought about 13' of 60mm x 3mm for a friend's new pole for less than $50).
Rivet your end fittings on, and Robert is, as tehy say, your Mother's Brother!