Pacific interests limit Four-Power membership

Could additional powers join a strengthened Four-Power agreement, and which states would be plausible?

Italy is a possible entrant, but the agreement's Pacific focus makes membership useful mainly to powers with direct interests in that region.

The proposed arrangement is a Pacific agreement rather than a worldwide collective-security organization. Its purpose is to manage possessions and threats in that region. Italy might seek inclusion for reasons of status under Mussolini, but it lacks the same direct stake as the existing parties. Geography constrains which alliances are strategically meaningful.