You're the one with the specification, so I just have to guess. It looks like (from your code) the switch is a single pole, double throw with the poles wired between pins 4 and 5 of port B and common tied to either ground or V++. Actually, I'd build it so that it was a SPST just wired between the two pins.
Presume that pin 4 is ground when the switch is pressed and V++ when it isn't. A normal switch is going to bounce for a while when it makes contact. I'd check pin 4 to see if it was ground. If it was, I'd check again something like 20 ms later. If it was still ground I'd set a variable to say that it was pressed, and turn on the LED. I would then check it repeatedly to see when it went to V++ (switches don't usually bounce on the break, only on the make). When it did, I'd set the LED off.
If your system does more than monitor the switch, then you'll want a non-blocking operation. This means making the check when you receive an appropriate timer interrupt or a hardware interrupt tied to the switch pin.
Again, this is just generic. You're the one with the parts and their specs.