Just rode it 2x this past weekend, we had FP+ for the first ride and were on and off in 10mins. At that time stand by was 120mins long ... so I would say get a FP+ if you can, otherwise you will be standing in line for a long period of time. We rode the 2nd time without FP+, made the mad dash with all the other park goers and got in line at opening, we were on and off in 35minutes.
As for it being suitable for a 4yo, there were alot of small children in line, I don't think its a super fast ride, it has lots of turns so you can feel the mine car sway but not scary, not at all. If you child enjoys the Barnstormer, then he/she will be fine on this ride!
It was alot of fun for us!