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.
There is actually no need to keep the test format as an active format of cricket, to be honest. No one is interested in this format and the teams are also not interested in playing test cricket anymore. At the same time test cricket is not something with which cricket is going to spread throughout the world as well. So I think this cricket is just a waste of time. There is no reason for only four teams to keep playing a format of cricket that no one actually cares about anymore. I honestly don’t think that the people who actually loved test cricket truly, even they actually care about this cricket anymore.