1. There are probably more cut scenes than Gameplay if you play on easy mode and just go straight through....but if you take your time trying to find everything, trying to be stealthy you'll find the amount of gameplay time can be pretty big....it doesn't matter that there's lots of cut scenes since the story is amazing and emotional...you will cry!!!!!!!
2. No MGS4 doesn't give you enough info about the events in past games....I would advise going to wikipedia and reading the story section of MGS1-3....shouldn't take you more than half an hour to do that. The plot is pretty complex..so if you have any questions ask them in this thread. PLEASE READ THE PLOT OF THE LAST GAMES....or MGS4 won't be as amazing to you as it was for many of us!
3. If you don't wanna read the plot of the previous games you can still play them ye know...MGS1 is on PSN...MGS2/3 should be easily available at your local store
4. I never played much on the online bit so I can't give you a proper comment there.
5. Welcome to the best game ever.