Transplanting is easy and not at all hard on plants if done correctly. Counting my vegetable garden, I've transplanted thousands of plants of all sizes, in all conditions, without anything I would consider a failure on the plants part. I as the grower have hurt a plant or three from mistakes, but really, I'm beginning to think this whole "transplant shock" thing is just another marijuana growers myth.
Roots are tough things, not wimpy and weak. Take steps to ensure they stay together without internal ripping and shearing, and you can easily remove that soil mass from the planter, take a large serrated knife, section the rootball into four pieces, and replant individually.
I like to do this when the soil mass is dryish. Water weight in the soil is one way to easily tear/shear roots inside where they can't be seen. If this happens up near the base of the plant, it may not be able to move enough moisture to live without damage and recovery time, and you might kill it.
When the planter is dry and lightweight, place the palm of one hand as best you can on the surface of the soil. Flip the planter over with your hand in place and invert. If the soil mass doesn't come out right away, you can shake it around a small amount, and/or set it back down and massage the sides to break the roots away. Try to invert and remove again.
When it's free of the planter, look at it and visualize each plant with an eaual amount of the soil mass, divided into four. Each section should obviously be centered by a stem. If two stems are very close together, you'll have to cut between them, and this can damage the plants. Two growing close together may have to be left to grow together.
Use your large serrated edge knife to cut these sections out lengthwise down the soil, like cutting a piece of pie out. It should cut through easily, with light resistance points when a root is being sliced. Have your four prepared new planters ready, half filled with soil, and more new dirt ready for fill in.
When you have one of the quarters in hand, insert it into the new planter and fill in with fresh soil. Some growers like to use Superthrive when watering into the new planters, It's specifically used for transplants, I've never used it and been fine, both indoors and out, but you might want to try it.
Water the plants into their new home thoroughly with a 1/2 strength fertilizer and set back in the shade to adjust to their new home. Set in the full sun in a day or two.