I have had quite a few Lee presses and had a lot of good use from them. Original turret, Loadmaster, two Breechlock Pro, Challenger, Reloader and Hand presses. I had the Loadmaster for almost 20 years. But, I modified several parts of the press to make it reliable. The 4000 and 6000 are updated versions of the Breechlock Pro. They address several of the shortcomings of the Loadmaster.
The biggest problem with progressives is the primer feed system. There is a tendency to flip primers, especially the small rifle/pistol primers. Just be aware that sometimes it happens. The new Lee presses have a different feed system than the older ones so not sure of their reliability, but, they look better than the previous types.
The big difference in the 4000 and 6000 is the size of cartridge you can load. The 4000 is really intended for pistols and short rifle cartridges (.223 length). I loaded for a .308 with the Breechlock Pro, but, it was a royal PITA. If you are considering regular rifle loading then go with the 6000.
Number of stations. Nice to have more....but. When loading for rifles I found myself running two steps. Decap, size, prime in one setup. Then charge, seat in another. I wanted a better feel for seating. If you are sizing and seating in same run, the force of sizing masks all the other steps.
For pistols I'd go all in one run. Size and decap, prime, expand/charge, seat, crimp. The extra stations on a 6pack would allow for a bullet feed die and a powder check die. These are VERY nice to have. Especially the powder check die. A great safety feature. A bullet feed die speeds things up quite a bit.
The biggest caution is double charging. It usually results from a short stroke. You will get situations where the process is interrupted. Usually due to a wrong size case, an upside down case or something like a .380 case stuck inside a .45acp. Also when a primer flips sideways (again a small primer issue). When a problem occurs I learned to unload the whole setup, toss the partial rounds and start over. Trying to 'fix' the problem usually just causes more issues, like a double charge.
What do I use now? RCBS Rockchucker. I only load for my three rifles and I am not pressed for time. I weigh and trickle each powder charge and I want to feel the seat pressure of each bullet. I use a ram prime on the press as well. Sizing and seating are adjusted to close tolerances.
IF I wanted to load high volume again I'd look at the Dillon 550, especially compare the primer feed on it to the Lee's. I might still get the Lee instead, but, knowing that the press might need a 'tweak' for optimal performance. FWIW, Lee's case feed system works really well, as long as you pay attention to it. Seems 'cheap' but it works.