I'm having a problem with the mag release on a 4566. The mag release button is very spongy and won't drop the mag unless I push very very very hard on the button.... Any ideas?

I can't tell from your description if the button is spongy (soft) or very very very hard.

Generic advice: Disassemble and clean paying particular attention to the spring, plunger, and recess. If spring is rusty or broken, replace.
On reassembly, screw button down until plunger is flush or just below frame surface when seated in button groove.

John

Like John mentioned, it could be some grit and gunk stored up in there. Good idea to take it apart and clean it out, then you can also adjust the nut so that the catch disengages at a more comfortable level.

To take it apart, lay the frame flat with the mag button facing up, and a wadded up towel or cloth underneath the other side of the mag catch. Push down on the small plunger next to the mag button to unscrew the button and take it off. Assemble in reverse, and how far you screw the button back on will determine when the mag catch disengages. It is recommended to screw it in until the small plunger is close to flush with the frame (I like mine just above flush w/ frame).

Grips off?

Yes, grips off, gun unloaded. Slide can stay on, I like to have it off though.

This is video #1 of a series by forum member BLUEBELLYYANKEE, the mag catch removal is right at around 3:00 mark:

Bad mag release spring. They sometimes fail/rust/lose tension. Id bet if you replace it all will be well. Regsrds 18DAI

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Looks like Midway has the mag catch spring in stock at the moment: Smith & Wesson Mag Release Spring S&W 3953TSW 4003TSW 4006TSW 4013TSW