It's time to change the brake fluid, and it says to use only DOT 4 but I'm having a hard time to find it. Tried several shops, all they have is DOT 3. Some older bikes use 3 or 4, so can I simply take 3? What's up with these numbers anyway? Are these fluids really different from each other? I know silicone based fluid is, but what's the difference between 3 and 4?