What dia. screws came with the mount? I would expect something like #8 or #10 screws for a 20" TV and may depend on the number of screws and the pattern. 1/4" lag screws is just way overkill. Unless you drill the appropriate dia. pilot hole first, larger screws can split a wall stud so be careful.
This is an exterior wall? If so, you'll only have 1" depth to work with. You sure don't want anything going through the exterior siding. If you remove a cover plate from one of the outlets, you can check the exact wall depth. If the mount is not an articulating type that extends out from the wall, you don't have to worry as much about leverage pulling the mount out either. Not sure what mount you have, but not all have the screws aligned vertically in one row in the center. It may help to attach a small square piece of wood to the wall first, then the mount (as mentioned above).
The horizontal centerline indiacted on the sticker on the wall suggests something possibly wider than a 3/4" wide stud in the wall. Maybe there is either a few studs side-by-side, or a wood square behind the wall panel. A stud finder may show an outline of it.
Just looked at our friends 20" TV and articulating mount today in their TT, and the mount is attached to an end cabinet wall (3/4" thick) with 4 #8 screws.