That 4 wins in a row consisted of playing 2 teams that are on average 26 games under .500. They won the first one by 1 in the 10th inning, the second one by 1 in the 10th inning, the third one by 1 run before finally winning a game by a decent margin (4 runs) today.
I don't think the Brewers are playing all that well right now. Of course, they way they are built (bad offense, good pitching) they will naturally play many more close games, and to their credit they won the games when they easily could have gone 2-2 or 1-3 in that stretch.
As it stands, the gap between the Cubs and Brewers SOS has only widened over the last week or two. Based on pure record, the average remaining Brewers opponent is roughly 5 wins better than the average Cubs opponent (82-80 team vs. 77-85 team), and if you go by Fangraphs remaining SOS, the gap is even larger at 7 games (83-79 team vs. 76-86 team). I think that difference in schedule is enough to cover the 3.5 game gap and the division will come down to the 6 H2H games.
This isn't really responding to anything specific in this argument, just general thoughts on Cubs vs. Brewers.