You can't take only 4 props to a world cup. I mean, I daresay you can by the rules, but consider the ramifications.
Every prop must tog out for every match. Zero rests. If a prop is not fit for a match, then he has to be sent home and a replacement called, because you have to have 4 fit props in the match day squad. What odds on one of our props having a ****le that keeps them out for a week? No one wants to be playing Brookes while Cole is fit because Cole had to miss a week or two.
Ultimately, you pretty much need 5 props. You need to be able to rest players properly, you need to be able to cover missed games within the squad. Really, you want 6 props, but I'm not sure how you do that in the current World Cup squad sizes, which should be changed to reflect the match-day squads being one man bigger with three positions locked in, but nothing's been done about that yet. And if you only have 5 props, one of those props is going to be ambiproptrous, because once again, no one wants to see Brookes playing and Dan Cole sitting at home. You can't rely on calling up players for every injury.
Taking only 5 back-rows is the obvious way of squeezing in that 5th prop with one lock expected to do emergency duties (dare I say it, Nick Easter might be being lined up for that sort of role).
Also, going with only two scrum-halves isn't quite as big a risk, but it's still up there. You want three; and four centres is a luxury, particularly when your fly-halves/wingers should offer cover there.
It's a shame we have no guys who can play 9 and 10 really.