I don't understand how to spiritbond and get tier IV materia anymore.

[ess-vid]

As is typical of the first revision of 2.xx patch notes, it's likely a clerical error and they meant i70 or below gear. i70 has always yielded a tier IV materia, with i45 to i69 items yielding a tier III, plus a flat 10% chance to yield a tier IV. What's likely meant by the patch notes in practice is that spiritbinding i55 items such as Imperial Operative Wristlets will have a higher conversion chance to tier IV than, say, a Rubellite choker.

[Nekodar]

As is typical of the first revision of 2.xx patch notes, it's likely a clerical error and they meant i70 or below gear. i70 has always yielded a tier IV materia, with i45 to i69 items yielding a tier III, plus a flat 10% chance to yield a tier IV. What's likely meant by the patch notes in practice is that spiritbinding i55 items such as Imperial Operative Wristlets will have a higher conversion chance to tier IV than, say, a Rubellite choker.

Checking english, german and japanese patch notes it's definitely i70 and higher that is changed.
That means i70 no longer guarantees Tier4.