Matt Kemp, ultimately, will answer this question himself or, rather, via his medical test results. If his left ankle is fully healed sometime in April and he's able to run and do the things that made him one of the best all-around players in the game just two years ago, he will be in center field for the Dodgers.
Even if it takes him a while to get comfortable, he will be left in the lineup to work his way back into that zone. Every day. No questions asked. He's that talented and the Dodgers have that much invested in him.
On the other hand, even if Kemp were to stumble into another injury-wrecked season, Andre Ethier wouldn't be on the field every day. A lifetime .235 hitter against left-handed pitching, Ethier likely would have sat out many of the Dodgers' games against left-handed pitchers anyway, in favor of Scott Van Slyke or somebody else who doesn't struggle to make contact against lefties.
Some people will argue that Carl Crawford should be the odd man out if the four-outfielder logjam ever actually materializes. Leaving Crawford out is harder to do than it sounds, however, as Crawford is among the two or three fastest Dodgers and will be asked to fill a specialized niche in the lineup as the No. 2 hitter behind Yasiel Puig.
Ethier, at this stage of his career, is a middle-of-the-order bat with roughly average speed. He's a good, solid outfielder and an important member of the team. But if you're looking for one of the four outfielders who profiles best as a part-time player, he's your man.