So as long as I don't finis this current playthrough, my stats as they are will still be if I reload the beginning of Act 5? I can deal with that. I'm considering reloading anyway, but it would be great if I can keep my current stats.
Oh, sorry, I misunderstood you. Basically you start the game at Ground Zero. When you begin a playthrough your stats are all at zero. As you play through the game, your stats will change and when you save the game your stats are saved up until that point, irrespective of what you do after. So for example if I am in Act 2 and I have x kills. I make a save, and I kill 500 enemies, exit and enter the area and make a new save (not overwriting the old one). On the new save I'll have x+500 kills. If I reload the old one however, I'll have x again. This, to my knowledge, holds true for every stat.
So it doesn't matter if you finish the playthrough or not. If you have a save at the beginning of the Act, your stats will be whatever you had when you made that save. Unfortunately, as a result, you won't be able to keep any stats you got after that save but I would say it's worth it.
@BigNastyCurve, If you want an understanding of the plot I would highly advise you to play Metal Gear Solid 2 and Metal Gear Solid 3 at least before you play Metal Gear Solid 4. Many people, myself included, don't think Metal Gear Solid 4's plot is as good as the previous games (and is quite disappointing) and for the gameplay you'd be able to enjoy it without playing 2 and 3. However, you should play MGS2 and MGS3 regardless, not because you want to move past them onto Metal Gear Solid 4 but because they are remarkable games in their own right. The sheer amount of options, areas to explore, items to find and collect and hidden secrets within those two games is enough to last for hundreds of hours. If you can, the first game is very strong too and although the graphics have, of course, not really held up, the gameplay is still perfectly playable (although adjusting may take awhile) and it too is a highly enjoyable game, albeit with less hidden content (that I'm aware of anyway) than in the later games so I do suggest you play through the entire series if you've the time.
Only at the end of every act. So create seperate saves when attempting BB run.
This is true but it's worth keeping in mind that you can make a save halfway through an act and then exit to the main menu where you can view the mission briefings (I always choose Act 2's when checking these) to check your stats as they scroll across the screen in the orange bar (just don't save after you do this in case it adds to play time).