1) Jerry is the restaurant's main cook and Tanya backs him up/helps him out when there are plenty of people.
2) Sonja said she would back me up/help me out if I complained about the new fill-in at work.
3) Would you mind backing me up/helping me out? I need a hundred bucks within next weekend.
4) We had a lot of work to do, so Maria helped me out/backed me up.

In these kind of sentences both verbs mean: to lend a hand, to give help to someone.
When these two verbs mean "to give a hand to someone", can they be used without distinction? Or are there any specific collocations to use with "back up" and others with "help out"?