4 equals max sockets on a 4 slot item but 4 does not equal max links on a 4 slot?

This will not be fixed as it is not broken. Fuses only fail to work at max links on 6-socket (and I believe 5-socket, someone will correct me if I'm wrong) items. It initially applied to 4-socket items, but this was very poorly received by players, many of whom had common cases where they needed to break a 4-link item into separate 2-socket linked groups or similar to prevent support gems increasing the costs of other skills in the item they weren't wanted on.
We listened to the feedback and changed the system - it's been this way for quite a while.