going from memory on the various b4s I have had, at some of the turnbuckle lengths the threaded part of the turnbuckle going into the actual cup may flex some at certain lengths. Is this the part that doesn't feel sturdy to you? If so, I found it to normal. You could use a longer cup, but if you need shorter lengths then that can sometimes be a problem if it grips too much.

Again, just from what I recall in the past with a variety of B4s I had.