If I were you, I would go ahead and play them as DT in your 4-3.
What happens is that if a 4-3 DT is traded to a 3-4 team, they are likely to be moved to 3-4 DE or NT. Or if an NFL team plays a 4-3 one year, but switches schemes to a 3-4 the next the player is likely to go from a 4-3 DT to a 3-4 DE. Occasionally you will have a true position change, like when the Chiefs tried to play Chris Jones at DE last year instead of DT.
What about drafting a DT last season but this year Sandbox has his position as DE3? I drafted him last season to play DT in my 4-3 defense? The player is Dalvin Tomlinson. Can I use him at DT this season in my 4-3 Defense?