Last four girlfriends I had never cheated on me. Call me insane, call me weird, but I tested them. She told me she was going clubbing (most recent and current girlfriend) and I told her "Alright sweetie, have fun

MWAH" Though what she didn't know was that I was going too. I was with a couple friends and I wore clothes that I never ever wore and styled my hair in a way I never did it just so she wouldn't recognize me. I've noticed a few guys tried to hit on her but she told em to **** off.
One test was good enough for me.
Though I guess, 4 years, I'm sure you guys developed a lot of trust for each other in those years. Just believe her and make sure it doesn't happen again.
Girls who take relationships seriously are hard to find these days.