We would recommend against using different particle sizes and stationary phases for guard and analytical column combination if you can avoid it. That being said, the Cogent 2.o™ columns are valuable so it may be worth trying and seeing if the 4um guard column does or does not have an adverse effect on efficiency in the method if you do not have an alternative.
If the purpose of using a guard column is to protect against particulate matter only, then a pre-column, Cogent Column Filter may be more suitable. Remember that a guard column is a “chemical filter” and could potentially remove or adsorb compounds from your sample.
NOTE: On a related note, a guard column with a different bonded phase should not be used with analytical columns except under certain circumstances such as lipid traps.