1. As long as the DOT 4 isn't silicone-base, it should be okay to mix.
2. Replace line one at a time, brake fluid in the reservoir will drain, but not that fast. You will have time to refill the reservoir. Use this method; empty the reservoir using turkey-baster or syringe method, refill with new brake fluid and start replacing one line at a time. Check reservoir after installing each line, top it off, then move to the next line. After all lines have been replaced, start bleeding procedure.
Do not let brake fluid linger on a painted surface, rinse with water immediately; otherwise the fluid would strip the paint. Use brake fluid from unopened container and make sure it hasn't been dropped or shaken.