I believe they are compatible. The DOT 4 container says 3 / 4 right? The only difference I know of is the DOT 4 has a higher wet and dry boiling point. I've seen resorvoir lids that say dot 3 0r 4 too.
good grief....I noticed that none of the naysayers said why you can't mix synthetic and standard, they just say you CAN NOT DO IT.
whatever. mix away. won't hurt anything, won't kill seals. that's a bunch of nonsense.
and by the way. I HAVE mixed regular with synthetic and haven't crashed yet!!
I should have posted in more detail. Yes, silicone is a definite no-no as far as mixing goes and yes you "can" mix the rest, but I highly recommend against it. True, you may never see a problem in a street car, but why take a chance? I flush mine annually to prevent/reduce moisture content, plus I always change it after a day of racing, and I exclusively use Ford DOT 4 fluid. Why? After 35 years of repairing and racing (IMSA, SCCA Can-Am and Trans-Am, club GT-1,2,3,4) I find what works and causes the least problems. I don't mix oil, coolant or gear lube either and it might be overkill, but I don't have problems and I've repaired other's stuff because of it. No offense to the others, but that's how I do it. The great thing about this forum, you get lots of informed opinions and you get to make a more informed decision base on that.