Oh dear - a four feet high hedge with Leylandii - well I do hope you bought an electric trimmer at the same time - you'll be having that out every month. These trees will grow at least a foot a season, and plant the recommended distance. Or I could give you the fashionable response to planting distance for these which is, one in Ireland, one in Scotland and one in England.... If you don't keep up with the pruning, expect a tree 100 feet high in no time....

Green Leylandi actually grow 3 feet per season once established(golden ones a bit less) but if you trim them with a hedgetrimmer in early September they are easy to keep in check. The top shoot grows the quickest and I would tip that out at the desired height with secateurs in late May and trim properly in September.

3 feet apart is the usual guidance but if you plant them say 2 feet apart they'll grow together much quicker and take nutrients from each other so won't grow quite as fast when older. I planted 18 inch tall leylandi 2 feet apart for a customer and she had a solid 6 feet high hedge 3-4 seasons later.

Your 3 feet tall plants will almost certainly require staking. Golden leylandi less so but green ones are a bit floppy in habit until they mature a bit.Do keep them well watered until late autumn. Conifers can maintain green foliage for quite a while after the root ball has gone dry but will die if they ever get totally dry but this won'tbe visible until it is far too late.

One tip. Tip out the trees about 6-9 inches lower than the height you'd like them to finally grow to as this encourages bushyness at the top and in subsequent cuts you can cut to the height you want.

Do not make the mistake of not trimming every year. The trees become leggy and not dense. Never cut into old wood( stems without green foliage) it will not regrow.

Laylandii have got an awful reputation, sadly as they can be very useful. All the same I'd be inclined to return them and plant something else like a yew. The reason is that they are susceptible to an aphid that render them brown in places and look awful. Also Yews will re-grow whatever you do to them. I have had a row of Laylandii for many years now that have gone brown and also I find it difficult to trim their tops. Rather than dig 'um up I planted two Montana's that have completely covered it and suppressed it's growth.....looks good too.

One of my clients has a leylandii hedge kept at about 15/20 feet, does a great job, necessary where it is, keeps down the traffic noise at the bottom of 120 feet of garden - but for a hedge that needs to be kept at 4 feet high, I'd have chosen a smaller plant, such as Lonicera nitida, Buxus sempervirens.

i think you need planning permission infact these days in a lot of counties because of there high growth rate so id personaly not use them but if you insist i would ask if your actualy allowed to myself .

No planning permission is required unless there is already a planning restriction on hedges/trees in the position required. However if a large hedge becomes a problem there are new rules and the council has to sort things out ( but as I understand it they charge for this) I'm sure a quick Google on this would tell more.

good thinking anchorman . it wouldnt hurt to ask the local council as there has been a lot of out of control trees and the restrictions you say are in town and city gardens i believe . you also need permission to have a paved drive now if its cemented due to flooding . you arnt allowed leylandi on my estate if they havnt already been planted.