Why would you expect him to drop off in 4 years? Yes, he's approaching 30, but for a post player (especially one who has never depended on speed/athleticism for success, 34 isn't old at all. Plus he's been consistently producing around the same level for nearly a decade with no drop off.
I see him as a perfect candidate for a long term deal right now. He's a proven performer, and at an age now where most of the issues he had outside the lines seem to have been out grown.
If I was memphis, I'd do all I can to lock up zbo, gasol, and TA. Thats a solid offense/defense core. I'm not too sure how I feel about Gays max deal, or conleys deal tho...If those deals come in the way of resigning Gasol it'll suck.