I only use monsoon tyres if the darkest blue bar is very very high. Otherwise it's a bit self-explanatory: if the bar is light blue and not full, I use inters and if it's something in between I use soft or hard wets.
I don't know but I guess the wet softs and wet hards work somewhat similar to the soft drys and hard drys. I. e. the soft wet gives you more traction and better grip, but wears out more quickly, whereas the hard gives you less grip, but lasts longer. I remember once losing out big time due to starting on hard wets - the AI usually, I think, uses soft wets, if the track requires wets. In general, as a tip, I would go for soft wets if the weather is changeable during a stint (which it usually is). You can see that on the forecast: if the bar, regardless of colour, is high, it means a lot of rain for that 30-minute period. If it's not high, it often means only a few laps of rain.
One thing to pray for, however, is the forecast to be right. I often find that the forecast is what it is - a forecast, and quite often I'm greeted with different conditions on the grid to what I saw on the screen before. Things then get nasty if I'm not on the proper tyre, but you can't help these situations really. Then the guessing game has a second round in that you can choose to pit after a lap or two to switch compounds or wait for the track conditions to change to something closer to what you expected (sometimes the forecast is right, but the predicted rain or dry spell arrives a bit later).
The weather system in GP4 (as is many other things) is still the best I've seen in any sim. That unpredictability I just laid out included. Be on the lookout for the dry lines to appear, be aware of when/if you deviate from the dry lines and time compound changes smartly, trying to make them happen near or at the time of your regular fuel stops.
Good luck and have fun.
(And feel free to ask more questions, of course, if something is still not clear.)
Edited 3 time(s). Last edit at 01/29/2015 07:49PM by Atticus..