I would stick with dot-4.

Quite aside from whether or not you'd get away with it, it's simply not worth the risk & let's face it, it's not expensive stuff.
Besides which, if you were to be involved in an accident (your fault or not) then I'm sure the insurers could use this as a means of wriggling out of a settlement..

 

my 2 cents is if the bottle has been around for a while chuck-it and get new anyway not worth it to try and save.

 

It's labeled Dot 3-4 because it is Dot 4 rated and backward compatible with the Dot 3 standard. It's fine. If it's an unopened can go ahead and use it.

 

Ditto. There is nothing "DOT 3" about it any more than any other DOT 4. Its just marketing addressing those who shop by matching numbers.

DOT 3, 4, and 5.1 (but not 5) are 100% backwards compatible. The primary difference is 5.1 boils at a higher temperature than 4 which boils at a higher temperature than 3. The trade off for higher temperature is a greater affinity for water. Seriously doubt you will be able to notice the water thing, but it makes the "use unopened container" advice more important for 5.1 than for 3.