Overall attendance counting each individual park yes Disney is still well above Universal. But on an individual park basis they are getting fairly close. Aside from Magic Kingdom the parks are all within a few million of each other. So in a hypothetical setting, if you took it out and only factored in AK, HS, and EPCOT and compare it to US, IoA, and assume a similar figure for EU if it was already there, you would have around 32M attendees at Universal Orlando Resort versus 37.5M. Still a decent gap but overall still fairly close. And looking at the water parks Volcano Bay and Blizzard Beach have very similar attendance figures. So the water park difference really only comes from Disney having two to factor in versus one.
If you look at each resorts growth over an extended period Universal has outpaced Disney on that scale. Here are the figures from 2001 (not looking at water parks):
US - 7.29
IoA - 5.52
Total - 12.81
MK- 14.78
EP- 9.01
AK- 7.77
HS- 8.37
Total - 39.93
So in the same time frame Universal has increased their attendance by 66%, whereas Disney has only gone up 47%. At that rate, while it would still take awhile, Universal would eventually reach Disney's numbers. And a new park is only going to help that more.
All this said I don't necessarily feel that Disney needs to add a 5th park to stay competitive but I think a focus on updating and adding to the parks as a whole and at a good pace is needed. And there needs to be more additions versus replacements, they need to expand offerings and not just update.