The lock button quitting is a fairly common thing on the galaxy s4. Without buying anything there isn't really a good way to get it going again. When those quit, they make the phone think the button is constantly being pushed. Thats a problem for obvious reasons.
I suppose you could remove the button from the board (this is a board mounted SMD component, a moderate level solder job) and boot it by holding the volume down (or up, I don't remember) and the home button as you plug in the charger. That would get you by but if you've done that much work, spend the $5 for a new button and install it.