That is a breather rather than an over flow... crankcase gasses back to intake to be re-burnt... Just like on a car or truck, these gasses are re-circulated as one of the emissions controls. Your oil level may be too high causing too much oil to pass to the breather. Check your oil level in BOTH places... crankcase dip stick and oil tank dip stick... follow procedures for checking oil level from your owner's manual.
Now, if you'd said that you were blowing blue smoke as well... THAT would be a sign of other motor problems. A 4 stroke quad should not blow blue smoke like a 2 stroke just firing up.
Chances are that you're burning off excess oil from too high an oil level. You'd probably want to check your plug as well to check for oil fouling.
"You lock the door and throw away the key. There's someone in my head, but it's not me."
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