Squeemish, you should seriously consider cutting that track up into smaller tiles. With a sprite that large you're allocating space in VRAM for a 4096x4096 texture. It's taking up way too much space, especially since you're only using the top portion of it... most of that image is wasted space.
Sure, it runs fine now, but when you start adding more graphics to your game you're going to choke your graphics card to death. Some people probably won't even be able to run your game, or they'll experience crashing due to running out of VRAM.
My advice would be to make smaller pieces of track that you can fit together like tiles. Not only will that save a crapload of VRAM, but it'll make things easier if you want to make new levels as well, since you can build new tracks out of the same pieces. It might take some work getting your custom collision masks just right so they line up, but it'll be worth it in the long run.
Oh, and make sure your tiles are a power of 2 square, and I wouldn't go any higher than, say, a 256x256 sized tile for a game like this.
Anyway, good luck with your game