My stock battery was starting to lose life (hard to start on cold mornings). I've had a Optima Yellow Top in my S4 for a year and I'm very happy with it.

Install was pretty straightforward. You need to use the supplied spacer if you want to retain the stock hold-down. I had to extend the + Red wire by using a 10" extension (because I used the 75/35 model).

If you happen to find the 34R (reverse) model, you won't need the extension.

I'll post pics later if people want to see them.

 

I have always been very happy with the RED tops. I have done some serious winching, and welding (using a ready welder), and it keeps on going!
Richard

 

The RED top is a very good battery if you dont overload the alternator. It can start damn near anything at any ambient temperature... But if you ever do start using significantly more juice than the alt can supply, you might want to consider jumping over to the yellow or blue top. I've used and promptly killed red tops doing that with my system...