I need to top up the brake fluid on my 2011 Subaru Legacy GT but I’m unsure what type to use. The manual states to use either DOT 3 or DOT 4 & there’s a warning underneath that states “Do Not Mix DOT 3 & DOT 4 brake fluids”.

I have no clue what type is already in it, is there any sort of test I could do at home to determine it?