My question is, will that really prevent teams and PU manufacturers from developing their PUs to any point? Or they might accept some penalty to introduce some more upgraded PU?
I guess it will depend on each PU perfomance and expected improvement for each particular development, so if Mercedes continue as the reference they probably will comply with the 4 allocations rule, but what about the rest if they continue a step (or several) behind?
I think as soon as they see some margin for improvement they´ll go for it even if that brings toghether a penalty, because currently grid position is not that important. Well it is if you´re fighting for victories, but for those who dont I think a better PU might be a bigger advantage than a better grid position. Depending on the improvement obviously.
So it will depend on each PU perfomance and manufacturer work at factory to find new improvements. For example I´d be surprised if Honda only use 4 PUs for the season, as they´re changing the whole layout (or a big part) and that means their learning curve is still on an abrupt ascend, so for them a penalty will probably be worth.
About Renault and Ferrari, I´m not sure, probably it will depend on how good their PUs are at the start of the season, but if I had to place a bet (and I have to, winter is boring

Toughts?