Definitely have some bad lifters, now what:

At least 4 of them are bad. Thinking I should just go ahead and replace them all. Does that make sense? Should I replace pushrods too?

Roller or flat tappet??????? What is wrong with them and what caused it???
You should definately replace all the lifters. If the pushrods are straight and the ends are good, I see no reason to replace them.

Roll the pushrods on a flat piece of glass, if they roll quietly they are straight. If they make a tap noise they are bent. I am also curious as to what's wrong with the lifters. If they are bad, check each cam lobe carefully for damage also, use a flashlight through the lifter bore.

Like Griff said, replace all of them. Are you pretty certain it's your lifters? Unless your running an agressive valvetrain, no reason to replace the push rods unless you find something wrong with them.