I've had my foot slide out from under me due to the floor being too dusty. I train in a garage, so that's always a concern. I've also had the hamstring pull due to factors other than what's immediately obvious. In your case, a good part of it was probably due to the foot slipping and suddenly loosing tightness on one side.
Paul Carter has a great article on setting up for the bench. Setting up like this will remove 90% of the causes of your foot slipping out from under you. The important parts (check list and video at the end of the article) are listed below:
- Start with your eyes under the bar and the bar at the back of the J-Hooks
- Tuck your toes as far under you as you can
- Roll the bar to the front of the J-Hooks and arch with your eyes under the new bar position
- Shove your heals down until they hit the floor (should feel like you are locking in place)
In this position, the only thing that will prevent you from staying in a tight set up is if the floor or your shoes are slick. If I'm unsure about the floor, I'll make sure the heals of my feet lock down so they touch the bench supports.
Uncommon causes of hamstring pulls/strains
- Insufficient sleep--in fact, too little sleep is the primary cause of most of my gym injuries. Mainly because it kills my ability to recover and I'm just more brittle overall.
- Insufficient food--either because it's been too long and my recovery ability is depleted, or because I haven't had the right kind of food and my leg cramps up. I now have something just before training, throughout training, and just after training. That seems to avoid this problem.
- Dehydration--dehydration causes all sorts of problems and even mild dehydration can set you up for injuries.
Since I don't know your routine, or how meticulous you are about your bench set up, I'm guessing what the cause of the foot slippage was. My guess was that it was due to a combination of a slick shoes or floor and not being quite as tight as you could be. The heavier you lift the more these little things become bigger issues.
You were lifting a 4RM (~87.5% of your 1RM), which is significant. Had you been doing a 10RM or less weight, you probably wouldn't even need an arch to control the bar and press it several times. I find that as I push myself ever harder that the little things matter more and more.