My team played its first game today and 4 players got injured. Is this normal? I'll need a squad of over 60 players if this keeps up.

Yes.

I don't bench those who say "probable" unless I have good depth at that position. You'd never field a full team if you did.

I have one Probable 4 weeks, two Questionable 3 weeks and 7 weeks and the other is Out 2 weeks.

Is there something I've missed in the help guides about injuries and the differences between Probable and Out for example?

The only time I bench a probable is during preseason

There is a good chance the Questionable will be listed as Probable for the next game, maybe even both. looking at his health bar can help to see how far along it is

If it's preseason, you might want to check your Misc gameplan options and reduce the amount of time your starters are playing. Until a player is fully conditioned he will have a greater chance of injury. I usually do the NFL-standard of 1st quarter for the first game, 2nd quarter for the second game, 3rd quarter for the third game, and 1st quarter for the fourth game and that seems to get a good balance between getting my starters in game shape without a lot of injuries.