Here are a few thoughts:

 - If you don't already, you definitely need to develop a vocal warm-up / cool-down routine.  You wouldn't run for four hours without stretching, would you? Why then would you sing?  I don't have time / space here to suggest specific exercises, but I'll say you should definitely start researching. A good exercise works from the back of the throat to the front. (Hint: search for Opera singer's blogs / advice about warm ups).  Doing excercises will improve your stamina, reduce the amount of damage, and will diminish recovery time between performances.

 - You need to drink water *all the time*.  Sipping / drinking helps a little during a performance, but you need to be drinking water all the time to be properly hydrated.  Without water, your vocal folds are basically just two pieces of dried play-doh rubbing eachother. 

 - You need to sing less.  You don't have to cancel any gigs, but just don't make your sets so long!  2-2.5hrs is a considerable length for a concert; 4hrs is a marathon.  Being hoarse is your body's way of telling you you're hurting your voice - whether or not you want to accept the fact.

 - Just like when you work your muscles, they get tired and inflamed.  So too do your vocal folds when they are worked extensively.  The hoarse sound you hear is the result of inflamed vocal folds inefficiently resonating with air.  This creates a distortion in the sound, which we perceive as "hoarse".

 - You see this all the time with inexperienced brass players.  They sit down, immediately start wailing out high notes to impress everyone, but an hour later, their chops are shot and they're faking it.  A smart brass player sits down, plays quiet, long, low sounds and gradually works up through the range; high notes then take care of themselves.  The same applies to the voice as well.