You just jinxed them for tonight
I think Painter's a lock so long as he doesn't follow the Abel trajectory. But if he is no better than Walker and Walker is consistent (and best-suited to starting, which he is) you can certainly make the argument.
It also seems fairly likely they will need six starters all year (off and on), so if Painter did go down it might not be for long. But I don't think Walker is truly ahead of him, and is just as likely to go to the pen as Painter is AAA. I think Walker starting as the #4 is just a respect thing and a less pressure for Painter thing.