Hello,
When I understand you well, up to 4 elements combined show desired behavior.
When 1, 2 and 4 patches (with feed) goe well, there must be something with the last combiner (that combines the 4 element arrays).
Place the 4 elements array together in there final configuration, and feed them from two sources (each 4 elem. array its own source). Check S11 and S21/12. If S11 is still OK (as with the seperate 4 element araays) and S12 is low, then the problem is in the combining, not mutual interaction.
Regarding isolation resistor. As the arrays are similar, they will show similar S11,S22 and similar S12,S21. Of ocurse assumming that you have strict production tolerances. In that case there will be no voltage across the resistor in a Wilkinson divider.
Other question, Did yuo also check whether current and current phase is OK for the 4 element array?