The only context needed is that Universal is probably going to wholly own any new parks. The only equity share that Universal won't own is the 50% of the hotels that are owned by Loews.
If you think of it in that context, then a non-Universal branded 4th dry park would be either SeaWorld Orlando (assuming Universal buys their Orlando resort), or something like Six Flags (assuming Comcast buys that company outright).
Comcast buying Six Flags would be a pretty interesting thing to discuss (control of DC/Looney Toon theme park rights in the US) as well as the potential for some kinds of annual pass deals that would look interesting in California and across the US. Six Flags also has some interesting projects outside the US that could be roped into it.
And in Orlando maybe you build the 3rd dry park as Universal's version of the Magic Kingdom with the 4th dry park as a Six Flags park.
But yeah, I don't think there'd be outside companies involved. Why would Comcast want to share their profits with outsiders?