Can we visit with friends who have a 4 hour layover before their international flight?

Yes, do tsa immigration and customs

If they exit the secure area, yes. But your friends will have to go back through security. For an international flight, your friends might have to budget two hours for that. But totally do it!

You need a US visa for the same. Also need to clear immigration which will take around 1 hour if there is rush.

yes I think you can.

Call their airline, as Security is a huge time delay.

Yeah, it's possible. Just enough time for a pint or two. :)