I just discovered a ~5mm l5-s1 hernia. i workout moderately at the gym 4 times/week-1h. is it true that flexing is bad in my case? how about crunches?

Disc herniation?: While flexing is thought to potentially increase risk of worsening a disc herniation.. It is not necessarily bad; especially if it isn't causing you more pain. I would be cautious with squatting exercises and recommend working on core strength (back/abdominals). Over overdoing repetitive flexion activities in the gym if possible as a precaution. Learn a Mckenzie program. Hope that helps!