if those 4 go, it will be a rushed Impeachement, then Trump gets removed by the senate before the governors can name the senate replacements. Shumer becomes senate majority leader. Vice President Pence takes over but the Republicans have no one in the wings to run for president. A real mess. I’ll bet Democrat Senators and all sorts of congressmen have done this too
Congress impeaches with a simple majority. The Senate convicts/removes the impeached with 2/3 majority.
I think it would be doubtful to remove these senators through the House impeachment and Senate conviction/removal process. However, the scenario you described is plausible and disturbing.