Even though Mass Effect Andromeda is the fourth game in the series, it’s not called Mass Effect 4 because it’s not a direct sequel to the original trilogy. BioWare has explained why it chose to go a different route.
As noted by TheWolfHall, Mass Effect Andromeda’s creative director, Mac Walters, said at PAX East that BioWare simply “wanted the trilogy to end“. This effectively means they wanted that story arc to have a conclusion rather than milking it for another game.
Andromeda steps around this since it’s set in a new galaxy. The game doesn’t have to reference anything with the original trilogy set in the Milky Way. There might be some Easter Eggs thrown our way for good measure, but that’s about it.
Are you happy with the decision that Andromeda is being made or would you have liked Mass Effect 4 to continue where the third game left off?