I am a returning player to IWD II. And I need some advice again on party composition.

Main Questions:
Is four party members or six better?
Do I need 18 CON for frontline fighters?
Could I use javelin's as missile weapons?

Thanks in advance!

I ran it with one of the pre-gen parties available, and those are by no means optimized chars, so that should answer most of your questions.

By the end of the game with a party of 6, my PCs were up around 15th lvl or so iirc. I don't think a party of 4 would be significantly higher than that without serious grinding for experience somewhere towards the end, since the exp-reward mechanic is elastic in this game based on your level.

You will find lots of arrows, bullets and darts. If you want to use throwing axes or javelins, you will mostly have to bring our own.

1.) The game was made and balanced for a party of six and I strongly recommend creating 6 party members, just give them scripts if you don't want to micromanage that much.
2.) No, but you need maxed STR for them and everyone else whom you want to do physical damage, else they'll feel quite useless and combat will be very slow. 13+ dex is recommended as prerequisite for combat feats.
3.) No.

Thanks for answering my questions guys!

I generally prefer smaller parties since they level up faster (I ended at level 18 with a party of 4 and had no troubles). It's really a matter of preference, and I think any party size is viable from 1 to 6, so take the number of characters you want to. Just be mindful that Wizards may be limited by the scroll availability rather than by level, so if you go for a really small party (3 or fewer) definitely use a Sorcerer instead. No. It's nice to have, but not necessary. I don't think there are any returning javalins, so you probably want to use throwing axes since there's returning throwing axes you can get.