I've done it a few times. It's not super steep, but super long. Issue is, there is actually a very small descent mid-way in the climb. So, the average gradient is NOT 4% in reality. It's a little higher if you take out that distance for that little pause in the action.
Next. Even Phil Gaimon took a good bit over an hour to ascend it from just past Marion to the top. If it took him that long, us mere mortals it's definitely at BEST a "Steady State" really low Z4 or upper Z3 effort. That's if you nail the pacing and can go deep in the hurt cave for well over an hour. So, I'd honestly just get a fluid trainer and do some 2 hr Z3 workouts once a week before you go.
Next. Ride a compact up it. I could probably get up it just fine now on my 53/39 that I'm skinnier and stronger.......but........to enjoy yourself have the best gearing ready you can.
Next. Check the schedule for the snack bar at the top. It isn't always open, and the cell signal can go to crap meaning no credit card transactions. So take cash and take enough nutrition in case you arrive at top and the bar is closed.
Next. I parked at the dollar store in Marion and bought a Gatorade there and just asked nicely if I could park in the far corner of the lot. Worked out well. Just time the weather properly. Low % chances of rain or storms, no fog, not 95 deg, etc.....
Enjoy! Try not to make it so much of a suffer fest to miss out the sights along the way. Lots of little waterfalls, overlooks, the tunnel. Oh, the tunnel. You have to have a front/rear light according to park service. You'll want it. You won't be able to see the road under you in the tunnel without a light, and there might be broken pavement or debris you run over if you can't see it.