[kernal]

The dealer can adjust the speed. Your cart is based on the golf cart so the drive train is basically the same. I just had this done on a RXV (golf cart). The technician plugs a small electronic key into a port near the speed controller. A couple of beeps and the "freedom" mode is unlocked and the speed is set at about 17MPH (on 18"tires). Then he plugs in a hand held "scanner" and can dial the speed up to 19.6 MPH. He can adjust other parameters also and may ask you for input. All this takes about 5 minutes and in my case was done while I watched and doesn't require the cart to actually be in the shop. In my area there would be no charge if you bought the cart from them. If you take it to another dealer or independent there would likely be a charge. I have never heard of this affecting the warranty. Because your cart has taller than 18" tires it will probably run in the 21-22 MPH range. We all would be interested in your final result. In my area the dealers set all the carts to their max speed routinely. Your experience may be different.

Bling: Sound options: 1- Radio with amp and speakers. requires power source, speaker mounting (possibly cutting the body), radio head mounting somewhere, and dealing with reception interference from the speed controller. 2- small amp and speakers with bluetooth. Still requires power source and speaker mounting but eliminates the radio head. Use phone and bluetooth for music source. 3- Get a quality mini bluetooth speaker. Use with phone. Nothing needs to be permanently installed in the cart. 4- Probably something I haven't thought of.
Lots of aftermarket accessories for EZGO golf carts. A lot of it will fit your machine but things specific to the body will be different.
Seat belts for the kids maybe? A lot of carts in my area have swim noodles fitted over the bars on the back seat. For kids protection, I assume but have no proof they actually make much difference.

The S4 is a good looking machine. Nice choice, I think.