Ok since apparently that works I’m gonna explain what I did so other people can do it if they want

I’ve been using tumblr app version 3.4.3 on an ios6 iPhone 3 since it was the most recent version (about two years ago) . I just changed to an ios9 iPhone 4.

Basically the old app was still in iTunes so I just put it on the phone like normal.
It consistently crashed a couple seconds after startup (tho it seemed to work fine otherwise). I put it on airplane mode (which stops it crashing somehow??), got to login screen, entered email/pw, then turned off airplane mode and clicked login before it could crash.

Basically the screens I’ve found it will crash on: dashboard when not logged in, activity tab, search tab.

Dashboard, making posts, liking posts all work apparently.