First, I assume that your file does not have a fixed name (as opposed to your question with a fixed name file.txt). If not, please explain why you need a time-based solution.
In any case, you can use a file mask with a time limit :
get "*.txt>2014-07-19 4:00"
, %TIMESTAMP%:
get "*.txt>%TIMESTAMP#yyyy-mm-ss% 4:00"
, , 4:00 (%TIMESTAMP#yyyy-mm-ss% yyyy-mm-ss, ).
get on WinSCP ( /command, /script ), % %TIMESTAMP%:
winscp.com /command ... "get ""*.txt>%%TIMESTAMP#yyyy-mm-ss%% 4:00"""
, : , , 6 . , WinSCP , / 2 (6 - 4 ):
get *.txt>2h
WinSCP .