Bambu Lab build plate holder, 4 slots

This is a quick remix of the 5-slot Bambu Lab build plate holder by @ag_atlpe_362886 – just with four slots instead of five. The reason for this is simply that this is the maximum number of slots that fits in the space I have available.

Note that I didn't remix the stand that would put this at an angle, and neither of the original stands fits this. However, I did leave the attachment points for the stand, so it should be relatively easy to modify the stand to fit this one if needed.