Maybe it's just me, but I don't see a reason for a 40-150/4 since we have some 40-150/175 with f/4-5.6. Ok, those are not PRO lenses, but at 210g my P45-175/4-5.6 it's exactly what this m43 system is about. One stop at the tele end from f/4 to f/5.6 is not worth, for me, any extra weight, size, money.
Oly makes wonderful camera bodies, but on the lenses side I seem to lean towards Panasonic (Leica). They simply seem to stick better with the m43 system's philosophy.
As for the "bright primes", the system is missing the f/1.4 primes. So Sigma has filled up the gap in the most economic/efficient way.
Right now if you want Olympus pro glass it is the 40-150/2.8. If you don't like the weight, there is no lighter option, other than the non-pro lenses, like the 40-150R.
Same situation existed in the FF world with the 70-200/2.8. That is a HEAVY lens. I tried it and passed on it. For me, it would have to live on a monopod. The 70-200/4 however at HALF the weight is much more usable, hand holding, for me.
If you do not NEED the f/2.8 aperture, the f/4 lens would save you a significant amount of weight. IF the 40-150/4 mirrors the 70-200/4.
Is f/4 worth it over f/5.6? It depends. If you are shooting in low light, then YES.
For me, it definitely was worth it. Rather than shoot at ISO 12800 with my f/5.6 lens, I could shoot at ISO 6400 with the f/4 lens.
I could shoot at ISO 3200 with the f/2.8 lens. But for me, the additional weight of the f/2.8 lens was too great for the extra stop of light. f/4 was "good enough," for the field that I normally shoot at.
As for Olympus vs. Panasonic.
Specifically the Olympus 40-150/2.8 vs, Panasonic 35-100/2.8. Yes, I agree. In fact, the 35-100/2.8 has been on my wish list for a long time, as a LIGHTER alternative to the 40-150/2.8, with the same max aperture. But it has a shorter max FL, so not quite comparable.