On the switches from global config= mtu size 1504, the 4 is the Dot1q tag, there needs to be MTU size alotted for the tag. Once both switches are equal on MTU, and a reload on the switches to get them flip over. You should be good, if not let us know. I do know that Dot1q tunneling will work with out a 1504 MTu, but I don't remember the specifics.
How to find the proper MTU size for my network | TP-Link Helpful hint: One way to verify whether if it is an MTU problem is to try and access the application or website via dial up access.Since dial up uses a default MTU of 576 bytes you will not have the same problems as broadband. If you have problems with both broadband and dial up access then the problem is probably something else. Ping Test to determine Optimal MTU Size on Router | Answer Nov 28, 2016
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How to Find Proper MTU Size for Network (with Pictures
Jun 27, 2018