Transferring trees from pots to ground.

I have a very small apple tree that was bought as a stick and has been in a pot for around 4 years. It's not very happy and feel it would be better in the ground. I've never got around to putting it in the ground due to health reasons, but do you think it would still be ok even if it's now root bound in the pot to plant it out and will it grow taller once in the ground?
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In the sticks near Peterborough
https://www.gardenersworld.com/how-to/grow-plants/how-to-plant-trees/
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.