If you see one in good condition, don't think too long. It will be gone!
You did not ask about gas or diesel, but here are my thoughts.
The red tiger engine was designed as a gas and was made a diesel as an afterthought.
The world series fords were designed as diesels and made a gas as a retrofit.
I own a 1964 4000 gas AP and a 1978 4600 Diesel AP.
The differences are numerous and great.
The cooling system is much better on the 4600.
The 8 speed crash box is vastly superior to the 5 speed. I do not like the reverse speeds though, low is too slow and high is too fast, the 5 speed is better here.
The clutch pedal is much easier to push on the 4600. A day on the 4000 makes my knee hurt.
Independent PTO is easier to use than the 2 stage clutch. You can engage and disengage on the fly.
The front suspension is much stronger on the 4600.
Brakes are night and day better on the 4600.
Faster road speed, 5 speed is SLOW to go on the road. 4600 still had the power to climb steep road hills without shifting.
Hydraulic system is much improved. More flow and control.
Both are 12 volt now as I converted the 4000 long ago. The 4 & 3 cyl 4000 would have a Generator while the 4600 has an Alternator.
Fuel consumption is much less on the diesel 4600 vs the gas 4000. and I get a lot more done with the extra power of the 4600.
I have manual steering on the 4000 (ARMSTRONG steering LOL!), the 4600 is one finger steering.
I added front weights to the 4600. Easy on and off with suitcase weights. I had to use the inside the wheel weights on the 4000. They are a PITA! The front of the 4000 still will come up with a 6 foot hog on the back.
I believe the newer models are more durable than the 4 cyl models. Time will tell on that one. They sure are heavier built.
I could go on......
I paid 6500 for my 4600. I would guess my 4000 at around 3000-3500. The 4600 is well worth the extra to me.
I hope that all makes sense and is helpful.