Since neither of us will be here when it happens, I’m willing to indulge in a bit of speculation on the issue, but is it possible that the ‘sealing’ of God’s people BEGINS with the 144,000 and spreads through the population of tribulation believers, or will the locusts have some ‘sense’ of who to go after? Since this judgement is from GOD, HE knows who are HIS. Just throwing an idea out there.
I think your interpretation of Rev. 7:3-4 goes beyond the clear meaning of the passage. It says the four angels will be told not to harm the land or sea until another angel puts a seal on the foreheads of the servants of our God. Verse 4 reveals the number of servants who will be sealed as 144,000. There’s no mention anywhere of anyone else being sealed. Using the same language as Rev.7, Rev. 9:4 says the locusts can harm anyone who doesn’t have the seal of God on their foreheads. This means everyone except the 144,000.