Start slowly. Don't think about spending four years of ALL your free time on this project. Think short term. 3-6 months and see where you get. Plan what you want to finish in 3-6 months and see if you are able to find the passion to finish what you planned. If you have the passion and it's coming along nicely, you may want to keep going. If, on the other hand, you're finding this work to be tedious and taking up time you could be doing something else with, then you have your answer.

Remember... if this becomes successful, your work won't stop at 4 years. It'll go on for quite a bit longer. Ask yourself if that's something you're up for. If not, keep this as a hobby project and draw your boundaries.

Good luck!