I upgraded my Rare Beta Gold Foil summoners to level 4 and 16 other cards today. I wrote about the reason behind picking each card to upgrade [here].(https://www.splintertalk.io/spt/@golddeck/upgraded-my-deck-to-improve-performace-level-4-summoners-and-the-cards-i-upgraded-to-match) So If you are planning on upgrading your deck soon take a peek at my logic about which cards to upgrade first.
Also shared it on twitter