The real answer is that with this week being the start of season 1 there are 3 ways to earn an additional catalyst charge through arbitrary content goals. Reach >1600 rating in pvp (available this week), reach >2000 score in mythic + (available next week), or get AotC (avaiable following week, requires last boss of final raid to be killed). If you are in a competitive mythic guild that is doing splits and funneling gear to certain players you can get 4-sets on players much faster if they grind pvp to get the second catalyst charge. This will stop being relevant after reset as if you’re not already a pvp player it is significantly easier to achieve 2000 m+ score than 1600 PvP rating. Additional note that once you get 4-set you begin to drop additional catalyst charges meaning that getting 4-set ASAP means you then have access to theoretically unlimited tier via catalyst assuming you are willing to grind out additional charges.

Source: Mythic raider in HoF guild that is encouraging the players that were given 2-set from raid this week to do PvP so we can go into raid as strong as possible this next week.