Does your mounts come with hardware I need M4 screws for tv?

Yes it comes with all hardware needed. Several sets of screws and sizes for different manufacturers. Also come with the large anchor bolts for the wall.

If i have a monoprice slim bracket already installed, will the holes in the wall match up and i can just remove previous bracket and install new bracket without drilling new holes? The previous bracket extends past the new widescreen TV, therefore i need a slimmer one but do not want to create more holes in wall..

The mount will not reach the studs in the wall, you would only use one stud and would need to use toggle screws for the other side.

How long are the mounting arms that hang on the backplate?

 Thank you for your question.

Those would be about 8.7" long.

I am looking at using this mount in my RV for a 32" Samsung LED TV. I don't have thick studs to screw into but I have a solid sheet of plywood behind the entertainment center. I am limited on the length of the screws to 3/4" but I can use several. Has anyone used this mount with short screws directly in to plywood?

 Thank you for your question.

Considering that his was designed to be used in wooden studs it would not be recommended to use this mount int hat configuration.