Biggest difference feature set wise is the automatic detergent aside from styling.
Some people love theirs.
It is a really smart idea as long as you use one detergent and softner pretty consistently.
If you change detergent often, use a lot of options (many prewashes), use much powder and so on, the way LG set it up might be working against you.
Changing detergent often means a lot of cleaning and readjusting.
It just can't dispense powder.
And especially with the LG, the automatic dispensers takes up your drawer space.
The 4200 has 4 compartments: prewash, mainwash, bleach and softner.
The 4500 has only 2 manual compartments: main wash and bleach.
So no once in a while manual softner dispensing, prewash only with automatic dosing.
The price difference is there (about 150$ from what I see), but not as much as one would think.
Wash and rinse performance will be exactly the same since they use the same wash package (software and hardware).
Pretty good performer apparently, however temperatures can be lowish depending on cycle and being the 5 cuft model spinning can sometimes take a while though that apparently got significantly better over the last few generations.