I agree with the above posts - there is a lot to consider when running a livery yard. With all due respect, 4 acres is not going to keep many horses. You would have to arrange alternative turnout/grazing when the fields need resting - clients would not be happy if they came on the assumption they had all year turn out, then found they had to keep their horses in as there was no grass. This would mean either you or the horse owner providing bedding, hay and feed, which all costs money.
You need to have good insurance in place, to cover yourself for everything that may happen. Injury to horses, owners, riders, visitors to the livery yard etc. etc. The fencing has to be good, and suitable for horses. Because there is only 4 acres, the horse poo would need to be collected daily, to keep the grass in good condition. Then there is the maintenance of the fields, topping the grass, rolling, chain harrowing etc.
As a horse owner I would want preferably a menage to ride in - floodlit for winter nights, or at least a flat field. I would also want good hacking from the stables, not on to busy roads. Would you specify all geldings, or all mares? Quite often on livery yards the mares and geldings are separate.
I would also prefer there to be a knowledgeable person at the livery yard, who would be able to feed my horses when I was unable to (happy to pay a little extra for this of course) be there for the farrier, vet etc. if need be. Also it would be reassuring to know they could spot when my horse was ill or something wrong, and call a vet.
If it was do it yourself, the client would be just paying for stable and grazing, and provide all their own feed and bedding. Do you have facilities for each person to keep their bedding and feed? What about keeping tack at the stables, and rugs etc? I would not be happy having a communal tack room, as you never know how honest the other horse owners are.
If it was part livery, then you would provide bedding, hay or haylage, and have to muck out the stables and turn out and bring in each day. For this you could charge a lot more. Again, have you the storage for the bedding, hay etc. And would you be confident handling horses, putting on and taking off rugs (most wear a stable rug at night and then have an outdoor rug in the daytime)
You also have to remember that people will be coming and going all the time ......early morning, late evenings. Probably bringing friends, children, etc. They may have a horse trailer and want to park it at the yard, will you have the room, and is there room for clients to park their cars.
I would also advise you to check with the local council re planning permission for a livery yard. If it is causing extra traffic on a road, or will affect neighbours, there may be restrictions. I know when I put in for my stables, I had to say it was not for a livery yard.
If possible go and visit a couple of livery yards before making a decision. That will give you an idea of whats what.
I am not putting you off, just want to make you aware that there is a lot to consider.
Perhaps the easiest way, is to advertise the stables for one person to take on, or two friends, and just do DIY, so they do everthing themselves.