You need to have all the DOT 3 or 4 out of the system. From what I've been told, even a little bit will cause issues. Biggest "problem" with DOT 5 is getting all the air out of the system. It can get tiny bubbles that seem like nothing but, added together give you a mushy pedal. Beyond that, DOT 5 is better and plus, it doesn't harm paint or absorb moisture, which is DOT 3 & 4's biggest downside.
In my case, it was a matter of not practicing what I preach and changing the fluid every 2 to 3 years. I'm an auto tech (since 1984) and know better. It's sort of the thing like the roofer having a leaky roof or the shoe maker with holes in his shoes. You do it 45 hours a week, every week, for years,,,,,,,,,,and years,,,,,,,,,,,and years,,,,,,,,,,,and at times, the thought of working on your own vehicles isn't really something you want to do.