Now...the very least thing that I want to do here is make matters worse and get you more confused. I am not "condemning" the use of 5. If you have that many cans of 5, you could flush the system with the 5 and use 5. You just have to get it flushed with one thing and use one thing all the time. The only reason I was recommending 4 is because it is much cheaper and more plentiful.
If you have that many cans of 5 on-hand, you can use a couple of them to flush with. 5 or 4 will work...BUT NOT BOTH!
To answer your question, there are a gazillion brake systems running around out there with DOT3 and DOT4 in them. DOT2 is practically obsolete. Based on what I have read, the 3 and 4 are actually better on the seals as the glycol fluids have more lubricity than the DOT5. In other words....no...the DOT4 will not harm a thing and your seals will simply love it. Just...for gods sake....don't mix the 3 or 4 with the 5. Use one or the other and let's just not mention 5.1 again to prevent that number from even coming up and adding to the confusion.
If you have that many cans of 5 on-hand, you can use a couple of them to flush with. 5 or 4 will work...BUT NOT BOTH!
To answer your question, there are a gazillion brake systems running around out there with DOT3 and DOT4 in them. DOT2 is practically obsolete. Based on what I have read, the 3 and 4 are actually better on the seals as the glycol fluids have more lubricity than the DOT5. In other words....no...the DOT4 will not harm a thing and your seals will simply love it. Just...for gods sake....don't mix the 3 or 4 with the 5. Use one or the other and let's just not mention 5.1 again to prevent that number from even coming up and adding to the confusion.