Laylandai help

hi, I'm a very inexperienced gardener. We have a large laylandai hedge, belonging to a neighbour and I'd like to know if I cut the branches right back to the trunk, will it kill them? I'd like to cut it right back and the build a wall up to about 4ft. I don't want to kill the trees at all or damage them in any way, but I would like to build a patio, with a wall and have the laylandai still grow above it. Also, if it is possible to do this, will the branches then continue to grow, and then eventually damage the wall? I hope this makes sense. I have no idea what I'm doing, so please forgive my ignorance.