On large dia WhiteOak, The 4in cylinder will work just fine for single splits. Will work pretty well with a 4way. If you plan on a 6way, I would go with the 5in cylinder. I use a 6way and 5in cylinder and altho I have never stopped it, It has had to really grunt a few time.
A 5in cylinder with more than a 2in shaft will be very expensive, while NorthernTool has a 5x24x2 for about $300. If you buy the Northerntool cylinder, be sure to drill out the ports before mounting it on the Hbeam. For some reason, their 5in bore cyl has 3/4 pipe ports, but has a 3/8 hole inside. Dumb design if you ask me, why weld on 3/4 ports when the tube only has a 3/8 hole. The larger shaft will speed up the retraction speed of the cylinder, but I would just go with a 22gpm pump and it will be plenty fast enough. Be careful when choosing a control valve, some only have 1/2in work ports, but you can find them with 3/4 work ports. The smaller port ones are only rated for 20gpm. Even if your pump is only 16gpm, you will have more oil flow going back to the tank because of the size of the shaft when you retract the cylinder. Get the valve rated for 25gpm, not that much price difference.
I also echo the statement about getting a oversize engine. When the pump has to deliver 3000 psi, it will bog down the recommended engine size for the pump you choose. This slows down the splitting action, even if it doesnt stall the engine. Going one or two hp bigger than recommended will help the engine maintain rpms and pump flow. I have a 28gpm two stage pump 5inbore cyl, and a 25hp engine. Motor never bogs and cylinder never slows. Splitting 6ways, I can split a cord of wood in about 15minutes and make 5 guys tongue's hang out doing it. Fast cycle times are wasted when its just me doing the loading and splitting.
Another thing to consider is you Hbeam. It takes a lot of metal if your going to do 6way splits. I wouldnt even consider anything less than a w6x6 hbeam. Look for a w8x8, with 1/2in flanges and web. Plan on reinforcing the web and flange with 1/2in plate. I have a w6x6 with 1/2 plate onn the flange and both sides of the web plated with 5/8 plate and boxed at the wedge with 6in channel. That 5in cylinder will bow and twist the Hbeam. I use a 24in tall wedge which puts extra stress on the Hbeam, but it doesnt leave a 30in dia round half split and hinged on top of the wedge.