Stock trailer matted and with shavings to absorb any urine. Small enough so the foal is not tossed around but not big enough he can get any speed if he tries to run.
We hauled foals straight off mama (pasture) for years like this. Never handled. We'd back the trailer to barn and scoot them in and then once home we'd back to our arena and let them loose and encourage them into a box stall.
At 4 months you are not going to teach them to stand quietly in a trailer. Their attention span is approximately 15 minutes.
For the 1.5 hour drive you shouldn't need to stop. Pay attention to braking, aceleration and turns.