Shomak:
I greatly encourage you to do tread searches for wood/log splitters on this site. You will find lots of great information, photos, Oops I wish ....
You will find a lot of folks are very opinionated at to what set up works best.
When I went to design mine I had certain requirements it need to meet. 1 - I wanted to be able to work while standing up straight. 2 - I needed it to be able to be pulled down the highway at normal speeds. 3 - It needed to have a way to lift the larger round off the ground. 4 - I did not want the wood to fall to the ground.
Basic Design (Ergonomics/Portability): To meet the work while standing up straight, I looked at horizontal & vertical splitters which had a waist high table. I liked the waist high table vertical splitters, but they would be top heavy to move around easily so i chose to go with a horizontal setup. I added out feed tables/wings to catch the splits and a hydraulic log lift "arm". I considered a davit/hoist with a winch, but liked the idea of using the lift to stage smaller round waiting to be split. (I can load it up before starting the engine or if I have helper(s) they can keep it loaded up & have cut my fuel use in half.)
Reservoir: Rule of thumb is 1 gallon capacity / 1 gpm flow of pump. This is to help with heat dispensation. You will need to allow space for oil to expand / contract as it heats/cools. If possible have the reservoir above the inlet of you pump. (This hint came from Brendan Casey) It will prevent dry starts & extend the life of the pump.
Pump: You said you have a Kohler 23. If that means 23 hp, you can run up to a 28 gpm pump. Since you are planning to use a 4" cylinder, a 16 gpm pump would be a good match for the cylinder. It will provide you with a reasonable cycle speed without rushing you.
Cylinder: You have a 4 x 24". You should have no problems. That is what I use & have no problems splitting elm or hedge.
Valves: Depends on what you are splitting & if you will be working alone. Most of the wood I have will split within the first 1/3 of the round. I use the log lift to stage the pieces to be split. With this in mind, an auto cycle valve (it has 2 levers you lock in place which allows it to extend the ram all the way then return it automatically,) would actually not be useful. (Plus it cost 2x as much as the "log splitter" valve I used. A "log splitter" valve requires the operator to hold the valve open during the splitting stroke but will lock in place for the return stroke then release. If you will be working alone and will be having to chase after the rounds with each one you split, then an auto cycle valve may be a good choice for you.
For the log lift I used a valve with spring returns both ways. On the base side of the cylinder, I installed an restricting orifice that I drilled out to 1/4" to moderate the speed of the lift. I connected the valves in series using an 3/4" black pipe nipple.
** Note ** Make sure the valves are designed to handle the volume (gpm) of your pump.
Wedge: Using a 4" cylinder, you will want to stick to a single edged wedge. If you plan to use a davit/hoist with a winch, you could use a "T" wedge. You will not have enough tonnage to use a 4 way wedge unless the wood you get splits real easy. My wedge is made from 2 pieces of 1/2" mild steel plate welded at a 30 degree angle & is welded to the beam.
Filter: To protect you pump you will want a filter. Make sure it will handle the volume (gpm) of the your pump. Install it on the return side of your valve group to help prevent cavitation. It is generally accepted the use of a screen in your reservoir is not needed & some feel it can cause problems if you have one.
Hope this helps! Have fun!!!
