The plastic trim is held in place by plastic clips. They will pop out as you pull downward gently. I found it easiest to pull it down from the rear edge where it meets the rubber seal of the rear window.
They are friction clips. They pop straight in and out. Just pull straight down from the roof. IIRC there are 4 clips.
Pretty much any 4" speaker will fit in there. There's plenty of room for the magnets unless you have some really deep unit. Don't forget to get your hands on the Isuzu speaker cable clips if you don't want to solder the new speakers in.
The front doors are a little tougher, you WILL need to do some minor tweaking to get anything other than stock in there.