Nice! :)
Yup, did it this way by design, the large dragons starting to appear from the 2nd year could appear like some final boss for intermediate players, then once you get the feel of the game's mechanics beyond Match-3, eventually you make it to the 4th year... It is possible to go further, pushing it (and now experimenting with the v1.1 add-ons while developing it) I have some worry that someone absolutely understanding the mechanics to its depth and playing it near perfection might possibly be able to "beat" it, in the sense that there is a point where the game runs out of being able to give you stronger waves. No worries, that's quite a way from the 4th year :)
Otherwise tried to design the difficulty curve and mechanics so it hopefully should give an interesting game for all skills, and at any stage of the game. Hopefully more or less works :)