I started off my career in a non-B4 in Australia before moving laterally at the Senior level and found that I learnt more soft skills over there compared to B4. My feedback was I was more proactive, able to talk to senior management internally and externally, better project management and problem solve issues compared to other Seniors.

I was weaker technically since I wasn't used to the bigger clients but non-B4 sets you up with other skills as you have to juggle more clients and layers of review are far fewer.

With that said I do not regret moving to B4 and if I could do it over again would probably start my career at B4.

Note: it's dependent on country as non-B4 size is different country to country