EX4200/EX4500 doesn't support MC-LAG. EX4300 supports it.
However, you would need two devices that support MC-LAG to use the feature.
So, in your case, this would not be applicable and you are left with the option of creating a ring between EX4200 - EX4500 - EX4300.
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If this worked for you please flag my post as an "Accepted Solution" so others can benefit. A kudo would be cool if you think I earned it.