DOT 4 is more hygroscopic than DOT3. Thats why in police pursuit vehicles they use only high quality DOT3 fluid. Water will absorb through the rubber in the brake lines, and the heat of high performance driving will cause it to boil and cause sever brake fade, even complete loss of pedal. Also after every pursuit or every 30,000 miles they bleed the fluid.
Its a good idea to bleed your fluid after a track day. I know its probably overkill but I bleed my fluid every time I change my oil, I do this every six months. With a pressure bleeder it really doesn't take that long, and rotate my tires anyways so its not much more work.
For a good fluid Castrol GT LMA is a high quality fluid and is readily available at most parts stores.
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