Yes, the primary broker can be either SQL or 4GL with -ServerType specified in the database start-up parameter.
However,
If Fathom Management/OpenEdge Management or Fathom Replication/OpenEdge Replication is being used the 4GL Broker is required to be started as the primary broker for the database (refer to the article# 000021731.
For Progress 9.1C or below the primary broker has to be used for SQL-92 connection, because the polling feature of the SQL-92 servers is only active for the primary broker (refer to the article 000001432).