If your tds WAS 140 and your tds is now lower (4) coming out of your r.o then unless you completley drained your tank before producing, your 'old water' is mixing with the 'new'.
In other words, at first, you'll have a bit of the 140 water left in your tank and you're adding water with a tds of 4 to it, creating an end tds of 15 coming out the brush.
After a few fills the tds in your tank will soon come down