changes on multimaster replication and activate GTID
OK, for anybody reading this in future.

I did the first step changing all server to use GTID based replication and it worked perfect.

I did it the way as I wrote in the original post.

Right now I have same landscape as before, just using GTID.

This night I'll go to activate circular replication between S4 and S5 
and if all goes well shutdown S2

I keep posting for the world after.