Reuben Reid pounced to give the Pilgrims a first-half advantage before Luke Young stretched their lead just minutes after the break.
Sheridan said: "We know who we were playing, a team chasing automatic promotion. But we were outstanding.
"I can only keep saying 'have a bit of confidence about yourselves' and we did that. We looked a really good team.
"To score four at Fleetwood is very good and we're working hard to keep a clean sheet. You've got players running 60 or 70 yards to protect that."
The win lifts Argyle to within five points of sixth-placed Fleetwood but Sheridan is keeping his feet firmly on the ground.
He said: "We've had a couple of results which have lifted us up the table but we can't get carried away.
"Two weeks ago we got beaten 4-0, things change very quickly."
- Sports & Recreation
- Conor Hourihane