splitting a string list

StringSplit[#, WhitespaceCharacter... ~~ n : NumberString :> n] & /@ lis

{{{"a (b)", "1"}, {"c", "2"}}, {{"d", "3"}, {"e f", "4"}}}

Here's an equivalent to kglr's solution, using replacement and regex:

{{"a (b) 1", "c 2"}, {"d 3", "e f 4"}} /. 
s_String :> StringCases[s, RegularExpression["(.+) (\\d)$"] :> Sequence["$1", "$2"]]
   {{{"a (b)", "1"}, {"c", "2"}}, {{"d", "3"}, {"e f", "4"}}}