Replacing moen 4600 valve
There is a "U" shaped clip that holds the cartridge in, it is sometimes hard to spot it. Use an awl or small screwdriver to get the clip out.

It is VERY common for these cartridges to get kind of stuck and be difficult to remove. They make a special puller tool specifically for removing them, you can probably rent the tool at a plumbing shop or good hardware store. The new cartridges come with a tool to assist in breaking the old cartridge loose.

I made my own tool to pull the cartridges out, it is just a flat piece of steel bar stock with a small hole drilled in it. I use the knob screw to fasten my bar to the cartridge stem and then just grab it and pull and twist.

Check out this video.