Brake fluid

I have just noticed the rear brake fluid reservoir is below the "lower" line and I'm assuming that I should fill it up before my rear brake doesn't work and I die. Question is, I don't know what kind of brake fluid was used previously (obviously DOT 4) but I don't know whether if it is bad I mix different-based brake fluids or if I should just get brake fluid and just bleed the system entirely. Thanks for any advice. via /r/motorcycles https://www.reddit.com/r/motorcycles/comments/dxg3hz/brake_fluid/?utm_source=ifttt