This clearly isn't a 7x7 crossword, so you want to combine the four smaller grids somehow to form a 7x7 grid of letters. Though, I stated that the diagonal was a 6-letter word, so perhaps we can expect a hole in the middle.
Notice some repeated trigrams in the grid:SUI, GET, BED, SCI
Flipping grids along their main diagonal when necessary, place the grids symmetrically in a 7x7 square, overlapping exactly at these trigrams. If done correctly, the main diagonal of the assembled grid will spell out ENIGMA.