Gap 4 licenses are not compatible for Gap 3 instances. 

Gap 3 licenses are compatible with Gap 4 licenses, however, but it will exclude the features that differentiate Gap 4 from Gap 3.  

GAP4 monitor license is not backwards compatible with GAP3. 

If the customer already has a GAP3 monitor license and installs GAP4 they will be able to monitor GAP their GAP3 project using the GAP4 program.