Not only could you do it with one $replace, but the way you have it now you will be editing the un-edited text each time. You should replace $1- with %r on the last 3...
Code:
var %x = some very long string
var %r = $replace($1-, bb, %x)
var %r = $replace(%r, tu, %x)
var %r = $replace(%r, pr, %x)
var %r = $replace(%r, bv, %x)