substring dynamic number of characters

substring dynamic number of characters

I'm working with 2 sets of data that were merged together, but they're
inconsistent in their format. Some are 10 characters, all numbers. Others
may have a separator : at position 4. I need to substring the first 4
characters. But if the 4th character is a : substring only the first 3
characters.
Does mysql have an IF functionality to determine the number of characters
to substring based on the character in position 4?
select substring(id, 1 , 3/4) from table1