DIY - rear seats removal & speakers
The only real diff between the gen 4 n gen 3 is the location of the things to pull down the rear seat. Also the gen 3 has those 2 screws in the middle of that back cover hidden under 2 stoppers.
Also ur big problem isnt going to be removing the old speakers or getting to them. But putting in the new speakers.
Your going to have to make an adapter with a medium density partical board. DONT bother drilling new holes, i tried it, ended up terrible. They wont screw even tights enough and will cause noise from the speaker when it vibrates since its not on tight enough.
i like 6.25 speakers more since perfect circle is better quality and its easier to make an adapter for.
Also ur other problem is gonna be keeping the speakers from hitting on the cover. You have to make sure the aligment is perfect or else the speaker will hit the top cover when it moves upward, causing unwanted noise. U have a lot more room for error with 6.25 speakers over 6x9 speakers for this.
Also if you want u can sand or dremel down the actual cover. Once you take it off, you can see the hole in it isnt nearly as big as the speak grill. If you expand that hole it will fix the problem of the speaker hitting the cover (if ur aligment wasnt perfect to start with)