That's because you really cannot compare doses between two different medicines. Morphine and Fentanyl are both good painkillers. 4mg of Morphine would be a reasonable dose. 4mg of Fentanyl would kill you, me, and probably a couple other people as well.
In order to compare different drugs, you have to compare equivalent doses, not equal. That 4mg of Morphine is roughly equivalent to 25mcg of Fentanyl.
The big problems with diclofenac, specifically, are a higher than average rate of anaphylactic reactions (unlikely [but not impossible] to be an issue if you've been taking it for years) and liver injury. Liver injury risk is higher in people who have other liver risk factors. Hence the need to know a persons history.