Even if it is the 5th most important event in tennis the gap between the Olympics and the actual majors is way too big for it to be called the "5th major".
I'm not sure if I'd put Olympics ahead of WTF. A few years back definitely not but after the games last year I'd say it's becoming the 5th biggest event.
Olympics is greater than WTF because of two key reasons:
1. It is held once in four years, making the chances to get the title less in a player's career.
2. Getting an Olympic gold medal is much more popular around the world than getting the WTF title. Non-tennis fans can appreciate the medal more than the WTF title. It's more prestige gained inside the biggest sporting competition on the planet.
Non tennis fans might even think that Olympics>slams. Recognition matters more when it's actually from people that know about tennis.