Out of the G4 members, India has the smallest economy.
Germany and Brazil should definitely get in. I doubt any of the P5 will dilute their own veto powers though, so it will probably be a permanent seat without veto.
But India's 1.7 trillion dollar economy is growing at 8-9% a year and it will overtake many economies by the end of this decade. It is not too optimistic to think that it would be fourth largest behind China, US and Japan by 2020. It would only take a few years after that for it to overtake Japan as well.
Germany would not make sense. It is an "old" country that is heading for geopolitical insignificance over the long run and really Europe should end up with a single seat, as both France and the UK will also become unimportant in the future.
Brazil is a good chooce with it's large, relatively developed economy. It has a massive population and huge land area, coupled with immense agricultural, energy and mineral resources.