Could the difference in the words, "shall be", and "must be", indicate that something significant happened in line 4:1?

Revelation 1:19 Write the things which thou hast seen, and the things which are, and the things which shall be hereafter;

compared to:

Revelation 4:1 After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter.

After John was called up into heaven, "shall be", was changed to, "must be".

Could it be that the timing between 1:19 and 4:1 was not connected to events, and was long, hence, "shall be"?

But once 4:1 happens, if that indeed is a picture of the blessed hope, then the timing of things will then be set?

i.e. three and an half years?

hence the use of, "must be", after that?