I'll try to go through each of the options you listed

1) Hold the Bitcoin

Sounds very logical. BTC has went up in value by about 900% in 2017 which means it's likely to keep on appreciating. There are many big institutions that want to offer BTC as a security to clients and some will be implemented in December, which will drive price even higher.

2) Invest in altcoins and leave a little in Bitcoin

Sounds even better. Diversifying your portfolio away from only BTC is the best way to make sure you don't incur big losses. You can hold about 2 BTC and invest the other 2 BTC is altcoins.

3) Fund a project and start my own cryptocurrency that solves a problem that current currencies face (scalability, Tx fees etc.). With the funds, I would be able to assemble a team, start a marketing campaign, get listed on exchanges etc. I am also a programmer which helps.

Better not. There are so many projects out there that unless you have a fantastic team, great knowledge and incredible knowhow, you won't be able to make a contrast with what is already out there. ICO's are really competitive and offering nothing does't really work anymore.

4) Convert it to fiat (probably the dumbest idea)

That's a second option. If you really need the money for something important do so. if not better keep it and invest in other coins or hold BTC.

Out of all the options I'd go for 2.