At 4 years I'm sure the batteries are reaching the end of their useful life. It wouldn't hurt to try the Resurrex but your best bet is new batteries. The problem is most likely sulfated or depleted plates.
I just had the same experience with 4 yr batteries and put in a new set. It's like I have a new cart.
BTW the best rejuventor I've heard of is EDTA. Here is a link that will explain:
http://www.webspawner.com/users/edta/
You can also Google "EDTA Batteries" for some additional info. Good Luck!