This is a short but truly beautiful game. Probably one of the most beautiful games on all of Newgrounds. The narrative is subtle but powerful, and the waiting mechanic is an effective and amazingly poetic undertone to the game. The true ending is sad, as you feel almost connected with the rock as you've spent basically +140 years with him in-game.
The music is absolutely breathtaking, peaceful, and serene, and the sudden shift in tone near the end of the game is wonderfully executed. It was also great to see how the civilization develops and expands over the course of the game.
The only suggestion I would make (which very likely won't happen, but just to point it out), to add even more beautiful depth to the story and game, would be to put in some civilians, or even just one, who you see after waiting for the stalagmites/stalactites: An old man who doesn't run away or panic when he sees you.
It would be a beautiful Easter egg for people paying attention and trying to get the 150-year achievement. I planted food for the people and got done with that with 376 years left, and after the stalagmite part, I resurfaced at 291 years, which would be around 85 years. It would be amazing to see one old man who remembers you and doesn't freak out when he sees you. It would add so much depth to the civilians. After a few more years, the old man would disappear, possibly with a gravestone somewhere nearby (aside from the secret gravestone, winkwink).
Absolutely perfect and beautiful game; 400 Years truly showcases that scriptwelder isn't just a game developer but, as his title says on Newgrounds, an artist.