I think you could definitely do both in 4 hours. If you're not doing the ice skating, waiting in line for Santa pics, etc., it won't take overly long to walk the lights (which are *gorgeous*, by the way!) You'd have plenty of time to visit the rainforest pyramid while the animals are still active, and then go see the lights.
BUT, that is generally speaking -- and November 29th is Thanksgiving weekend, so there will be a lot of out-of-towners. It's also the Holiday Gift Market there at the Moody Gardens convention center, which is popular with locals who then also go see the lights.
On the plus side, that night from 6-8pm is the holiday lighting celebration at the Galvez. I've never been to that, because I'm allergic to crowds

, but it sounds really neat! And free!
"Performances from The Nutcracker, handbells, bagpipes and Christmas carols will entertain guests at the Galveston Holiday Lighting Celebration at Hotel Galvez & Spa, A Wyndham Grand Hotel, on Friday, Nov. 29. This free family event begins at 6 p.m. and marks the start of the holiday season in Galveston.
The Holiday Lighting Celebration begins with the arrival of Santa Claus by horse-drawn carriage, followed by the lighting of the hotels 35-foot Christmas tree on the front lawn. Dickens on The Strands Queen Victoria will also make special appearances during the festivities. Additionally, children can have photos taken with Santa and receive a small gift."
So that might siphon off some of the Moody Gardens crowds for you. There is sooooo much that goes on on the island for the holidays!!

Go here to see all the different events:
http://www.galveston.com/holidaymagic/