Just potted four small fruit trees in large tubs, each morning the soil is frozen, will this kill them because I cant water them.

Can you put a thick bark mulch on top and/or wrap the pot in bubble wrap and fleece the top?

The fruit trees are hardy, leave them alone. Don't fleece the frost in.
You have protected the roots under the soil, too much protection will start premature growth.

~i wasn't advocating fleecing the frost in~you would have to do it when the frost has given~it will depend how small your little trees are and how badly they are being frosted,
My daughter has two conference pears about 5 foot tall in large pots against the back wall of the house and they made it through last winter without problems.
A single layer of fleece overnight is what Wyevale do with their plants.

I put some new ones in to join the dozen+ I already have just a month ago. The ones you have are fully hardy and will be fine. You need no fleece or mulch. Don't worry about watering. They are dormant.

John.