Hello @aarom and welcome to the ST Community :smiling_face_with_smiling_eyes:.
The PB4 is internally pulled up after Reset.as you can see on the page 72 of DS12288 Rev 6:
"After reset, these pins are configured as JTAG/SW debug alternate functions, and the internal pull-up on PA15, PA13, PB4 pins and the internal pull-down on PA14 pin are activated."
So, this is normal.
Best Regards.
STTwo-32
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