Great question amigo. Here is the whole truth and nothing but the truth. If you want, you can do the trip without 4x4, some parts might be challenging in a 2x4 vehicle, like the way to Soak Naala which you haven't seen yet.
In all honesty, I drove in 2x4 high for 80% of the trip, however, to keep things running smooth, and since speed didn't matter with the mogs there also, I kept 4 low on without locking the hubs of the front wheels. This gave me lots of torque, double the torque, without compromising steering turning ratios, and didn't keep load on the engine at all, kept it more on the gearbox.
I didn't HAVE to do this, I chose to. Also, if you plan on crossing bara paani, you will need 4x4.
Just remembered, there was a huge landslide somewhere near Naran I think, it was huge! And it was raining cats and dogs, with lots of slush and slippery mud. I chose to engage 4x4 there as I didn't want to risk a loss of traction.
Can the trip be done with 4x4 if you are a half decent driver, yes. On the tracks, but we went off quite a bit too.