If that is the case, then only probably the top four teams will be able to play test cricket. I don’t think there is any necessity for a different cricket format just for four countries to play. And my argument is that if people are only watching these four teams playing test cricket, either the other teams have to get better very quickly. Or there is no need for this format at all. I know a lot of people still watch test cricket. But that is a minority of the population that actually watches cricket. People don’t even watch one day international matches for seven hours now, let alone test cricket.
Apart from the pig-4, I guess Pakistan and South Africa also have the ability to play test cricket in a competent way. But still I don't know whether they will be able to form separate squads for test format and limited overs format. Anyway, apart from these 6 teams, I don't think that any of the other nations are capable of staying competent and playing consistently in test format. And there is no point in having meaningless one-sided matches, such as Australia vs Bangladesh, or India vs Sri Lanka. These smaller countries should concentrate on limited overs cricket.