I currently own and use a Lyman 450, Saeco, RCBS and four Star Lubrisizers. Each is used for a particular chore. The Star is by far the fastest, but it won't seat gaschecks, though some have tried, with limited success. Of the others, I like the Saeco best, primarily because of the side handle design and the ability to get size dies in .0005 increments for most popular calibers, so experimenting is easy with it and it will seat gas checks.
As for the difference between the Lyman 450 and the 4500, I believe they finally did away with the pressure screw through the bottom of the machine, which always ended up leaking when using high temp lubes and gets lube all over the place. I looked at one briefly and noticed that the base appeared solid, but I don't have one to compare with now.
With the Star machine, you use a flat push rod and run the bullets through the die upside down, so you're pushing on the base, instead of the nose of the bullet. This simplifies things because you don't need a different top punch for each bullet, like the other machines.
Sometimes it's hard to find a top punch that perfectly matches the bullet, though I have modified some by using a top punch from another shape and using epoxy to form to a bullet, after coating the bullet I use for the form with wax, so the epoxy won't stick to it. I chuck the top punch in my drill press, with a small amount of quick setting epoxy inside it, and the bullet on the flat drill press table and use the press to perfectly align the punch with the nose of the bullet. Of course the drill press isn't running when I do this, as it's only used to align the two parts. You also need to make sure the base of the bullet you use for the forming has a flat base.
You won't go wrong with any of the above machines. It all depends on your needs.
Hope this helps.
Fred