I want to know how to interpret this. I researched a bit but only read about multiple voices, which are obviously not present here.
The whole piece is in 4/4, only this one measure contains 5 quarters. Maybe it has something to do with the triplet?
as addition, here is a simple score example to illustrate the counting of 3 quarter triplets against straight quarter counting in a 4/4 time signature:
in your special case the rhythm would be as follows: