Moving apart to keep four oddly in proportion (3,4)
I believe the answer is:
pro rata
'in proportion' is the definition.
(I've seen this before)
'moving apart to keep four oddly' is the wordplay.
'moving' indicates an anagram (letters are moved around).
'to keep' means one lot of letters goes inside another.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
The alternate letters of 'four' are 'or'.
'apart' with letters rearranged gives 'prata'.
'prata' enclosing 'or' is 'PRO RATA'.
(Other definitions for pro rata that I've seen before include "Proportionally" , "Proportionately based" , "In proportion (Latin)" , "fairly divided" , "maintaining correspondence" .)