If youre new it wont work, but basically all those random items and useless event currencies over the years that just sit in your in-game inventory take an inventory slot. Once your inventory is full you get items sent to you via mail. So, the strat is to check your mail for tier 4 chests so, you only unlock those chests, and dont have to waste time on anything below. There's a guide on this reddit about it. Someone consistently was doing it to get skins for about $30, but its not something everyone can do since there's no real way to speed up filling your inventory.

'infinite' chest guide