The first four trumpets (Revelation 8:7–12) form one set, as is clear from the parallels between how the reports of these four trumpets are written up. Revelation 8:13 forms a break between the blowing of the first four and the remaining three trumpets.
7 The first angel blew his trumpet, and there followed hail and fire, mixed with blood, and these were thrown upon the earth. And a third of the earth was burned up, and a third of the trees were burned up, and all green grass was burned up.