You must have the same major version of IOS on all 3750 (otherwise yhey will not join in the stack), and the best would be to have the same IOS version on all 4 switches.
Read:
http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html#softcomp
Switches with different major version numbers are incompatible and cannot exist in the same switch stack.
Switches with the same major version number but with a different minor version number as the stack master are considered partially compatible.