Without notice, one keeps just some back (3,2,4)
I believe the answer is:
all at once
'without notice' is the definition.
(thesaurus)
I can't explain the rest of the clue.
(Other definitions for all at once that I've seen before include "in unison" , "Without warning" , "Simultaneously - immediately" , "How daffodils appeared to Wordsworth" , "Together" .)