A 4' Acer at that age will be easy to move. Just make sure you get nice rootball with it. It's not technical or complicated. Your Acer won't have too much of a rootball. You dont say which Acer you have but I'm assuming it's one of the Japanese types.
I moved Acer Garnet last autumn after it had een growing there for 3 or 4 years and was prob in excess of 5'......I trim mine for bushyness so it would have been taller ....but it moved with great ease. It will grow next spring unaware it has been moved,
I have moved several...one way or another...and they are one of the easiest of shrubs to move.
I really don't think you need to dig a trench......with a spade simply slice down at about 40 cm away from the trunk all round and lift. Simples! It will lift easily.
I always prepare the recipient site first. Don't replant too deeply ....simply keep at same depth as before. If you can add some ericaceous John innes to the planting mixture too it will help. Water well but then not again until spring when I would water well again and keep an eye on it to make sure it doesn't dry out.
Good luck dannyboy