I would prefer the 4 bay as the 5.25 drive bay is more common for standard pc cases. Otherwise, I would have to get a 5.25 to 3.5 drive bay adapter, which would not look as nice.
The 3.5 bay would be great for smaller pc cases holding a matx/mitx boards.
I would use this for backup purposes and having a different OS on other drives. The individual power buttons allow the user to be able to turn off a hard drive and build another OS on another drive without worry of overwriting or detecting another drive by mistake. Otherwise the user would have to open their case to unplug hard drives or hassle taking out drives from the caddies. This model is much quicker to work with.
One con may be appearance. It may not look as nice with drives sticking out of the bay. Especially different model drives that are different shapes and color (I would use all black Samsung EVO PROs). Maybe a snap on cover plate or lid could be used. I would also put space somewhere to allow labels/stickers, maybe next to the numbers 1,2,3,4 of each drive slot.