So I assume that Splynx makes a router and presents data streams with a single ip (as if doing NAT) and therefore the PCC system does not distribute the load on the 4 Wan.
This is incorrect assumption. You are blaming Splynx to be responsible for something that it's not responsible. Splynx is not involved in routing and creation of PPPoE session. It only authenticates the PPPoE username and password over database and says to Mikrotik - OK, customer can login. You can even say in Splynx service section to not assign IP to client, and IP will be assigned by Mikrotik from it's IP -> Pool section.
May I ask you a favor - for testing purpose, please try to disconnect Radius server or Splynx and just create static secrets in your Mikrotik settings on PPPoE Server. And use static IPs or Mikrotik IP pools.
What you will see - the same behaviour like when Splynx assigns IP address ?