Engine-wise, no, there is no real limitation. That was set at 4 purely for UI purposes, to keep it simple. Currently if you need more than 4 layers the only option is to use multiple images (yeah, not ideal for some uses).
There are multiple UI issues with expanding beyond 4, and all of them could be solved, but require more room. The current interface was not built to be very flexible (originally being an in-house tool just for me
), but with more users comes more use case scenarios, so yes I'm gradually working towards that kind of stuff.
More layers would require that they be nameable (too many and you forget what's what--even 4 can be tough to remember!) and more room in the UI and/or scrollability. Overall a good implementation would be a lot of work. I'd love to do it, but I'm too busy right now.
Support for more layers is planned, but it would come as part of a "flexible UI" update that will allow you move tools, and have optional parts of the UI as you see fit.