brake fluid

The level of the brake fluid has dropped below the minimum level - prossibly after I braked hard on the motorway to avoid a car that swerved into my lane without looking. I do not see any leakage around the liquid container, tubes or wheels.

I was wondering if I could top it up with DOT 4 brake fluid and see if it loses fluid again or if I should get it checked right away at a garage and drain and replace the fluid.

Is there any issue with adding new brake fluid to the old already in the system?

Thanks for your advice.