The 4 does seem to be a long route; with a 90+ min peak running time it's right at the edge of what I'd personally find acceptable for operator comfort.
Throwing this idea out of left field out there... perhaps the 4 should terminate at the 35th Street Red Line Station? Some remarks:
1. The South Side Red and Green lines, while not empty, are not close to capacity during peak hours. The marginal cost of accommodating customers on rail services is lower compared to buses. In contrast, the 4 takes about one hour round-trip between its King/35th and Illinois Center timepoints. At a six minute peak headway, this would free up very roughly about 8-10 buses (subtracting layover and the small additional distance west to La Salle St).
2. For intra-neighborhood customers, on-time performance would likely increase as the 4 would no longer be subject to CBD traffic, especially on Michigan Ave.
3. The freed up buses could be used in part for a complementary x4 route (Cottage Grove-Pershing-Lake Shore-Columbus-Balbo-Mich) during rush hours. A peak hour express route, akin to the 14# series on northern Lake Shore, would siphon off peak-heavy CBD customers while leaving the 4 with a more consistent demand and a 'flatter' schedule.
Regardless, even if the above wouldn't work bus lanes on Michigan Ave are sorely needed to keep buses on time.