Why did he not just take his existing 14 months of food over to the "pre mission" supply/launch vehicle, that has three years worth of supplies (for six people by the way) and just wait for the next group of astronauts to arrive. Then figure out how to get seven back instead of six. Also in that four year wait he could have figured out how to communicate with Earth and let them know he was still alive.
That was exactly his plan - his first course of action was immediate survival (water & food)
So that leaves enough food to fee six people for fifty days. I'm just one guy so it'll last me three hundred days. And that's if I don't ration it. So I've got a fair bit of time.
He only has enough food for roughly 300+ days - so the water making and planting buy him more time, which then lets him plan more complicated actions.
By Sol 7 he is already thinking of Ares 4:
There'll be humans back on Mars in about four years when Ares 4 arrives (assuming they didn't cancel the program in the wake of my 'death'). Ares 4 will be landing at the Schiaparelli crater, which is about 3200 kilometers away from my location here in Acidalia Planitia. No way for me to get there on my own.
By Sol 63, he has had a chance to re-evaluate his resources and reconsider going to Ares 4
It's time to start thinking long term. Even if I find a way to tell NASA I'm alive, there's no guarantee they'll be able to save me. I need to be proactive. I need to figure out how to get to Ares 4...
This is going to be a research effort, with a bunch of experimentation. I'll have to become my own little NASA, figuring out how to explore far from the Hab. The good news is I have lots of time to figure it out. Almost four years.
During a CNN interview (page 140 in the ebook version) Venkat Kapoor clearly states that while the MAV for Ares 4 is already on Mars, none of the presupplies are there yet:
“Do you think he’s planning to go soon? He seems to be getting ready for a trip.”
“I hope not,” Venkat said. “There’s nothing at the site other than the MAV. None of the other presupplies.”
A bit later (page 148), Venkat is speaking with Teddy:
“Problem is, the original plan was to launch presupplies a year from now. They’re not ready yet.”
So the assumption about the supply/launch vehicle having plenty of supplies is wrong (also, where does the "three years worth of supplies for six people" come from? Ares missions have food for six people for 56 days - page 19).
Can we figure it out from this? The manned trip to Mars takes 124 days ~= 4 months (possibly the unmanned trips are faster). There are "a total of fourteen unmanned missions", "start to finish, including supply missions, a Mars mission takes about three years", and for the MAV "it takes twenty-four months to fill the tank" (later we get a better estimate of 18 months). (All of that is in the first entry "Sol 6".)
So the MAV has to launch 22 months (at least) before the humans. The mission is 1 month, the return 4 months (if symmetrical, which needn't be the case). So if the overall mission is 36 months the humans get there in at month 31 (and launch at month 27). Thus the MAV has to launch at month 9 at latest. What happens during the earlier 9 months? Perhaps some supply (food and other materials) missions go then. And maybe some go immediately after. So in that case there'd be plenty of food around (he'd just have to go collect it by himself).
On the other hand, if it only takes 4 months for the supply ships to get there maybe they wait until after the MAV is there and working and only then send the later 13 missions - to get there before the humans launch. Why waste them if the MAV face-planted and busted up? So, launch the MAV at time 0. At month 4 it lands successfully (or not). By month 10 it's generated 6 months of fuel and is still going strong. Now you have a full year to launch 13 more missions and have them all get there successfully before the Ares 4 leaves earth orbit. So maybe the food isn't there yet (since the MAV arrives when Mark does the first food shipment on this schedule wouldn't arrive until he's been there 10 months).
(I realize this isn't an answer, as it is inconclusive. But its too long for a comment ... and maybe someone else can work on it and improve it. And check my math. And reconcile this with Mark's comment that he has 4 years to survive, while the entire Ares4 from start to finish ought to be 3 years.)