I have multiple (more than 4) Heroes with 4+ star level 16 gear equipped and I am unable to get credit for the challenge. I have even tried equipping another hero with Level 12 4+ star equipment and it wont advance beyond 2/4 complete. 

I also cannot get to the support page from the game, I had to go to the internet site to submit this. 

I have not had issues completing any other quests, challenges, or BattlePass missions. 

I am playing on Bluestacks 4 (Galaxy 8+ emulation)