I picked this up from a nursery. It’s about 4 feet tall. Could I turn this into a bonsai? What first steps would you recommend?
It's almost always possible to turn a tree into a bonsai.
The real questions are:
How long do you want to spend on the project? Note the timeline in the post above. And that 5-10 years is just the growing part. Probably allow another 5-15 years after that to build branches and ramification before reaching show quality. You may be happy to settle for lesser quality (see below) and might be able to get a regular 'mallsai' in 3-5 years.
And, what quality of bonsai would you be happy with?
Bonsai come in all shapes, sizes and quality. From 'Mallsai' sticks in pots through to beautiful bonsai that look just like real trees but smaller.
The lower branches are reasonably close to the surface roots which is a plus when it comes to reducing the height of the tree.
Most deciduous bonsai are created by repeated grow then chopping to a side branch to give trunk bends and trunk taper.
For a simple 'mallsai' you could chop the trunk and branches and continue trimming as new shoots grow. Over 3-5 years you should be able to develop a trunk with a rounded crown of foliage.
For a better quality bonsai you'll need to put in more effort over a much longer time frame.
Nebari (where surface roots emerge from the trunk) is important for quality deciduous bonsai. Most experienced growers recommend starting with the roots because that's easier when the tree is young and roots may still be flexible. Starting with the roots may mean complete bare root, severe root prune and possibly growing the tree on a board or tile for a year or 2 to get a good, well spaced and well ramified, horizontal root system that will make your bonsai appear old and stable. Please note that this sort of root work is dome late winter, before Spring growth begins. Doing much to roots now will probably kill your tree.
While the roots are developing we'd normally allow the trunk to thicken. There are a few ways to do this but usually involves letting the trunk and branches grow freely for up to 5 years, maybe with occasional chops to help with trunk taper and trunk bends.
When the trunk reaches satisfactory dimensions you'll start trimming more often and maybe direct some branches with wire to achieve a good looking main framework then continue trimming and shaping to develop the overall shape and fine twiggy ramification that makes a great bonsai look like a real tree.
For this year I would just care for the tree as it is. Learn to water and feed to keep it alive and healthy while you do more research on what you want to achieve and how to go about it.
Not sure how suitable your area is for Japanese maples. Texas has a reputation for being hot and dry, neither of which Japanese maples like but I note your hardiness zone is 9 which should be similar to mine and they do OK here except for summer leaf scorch if I'm not careful enough with watering and shade.
