the check engine light will come on to indicate knocking/pinging...its not good. The factory warning is set to come on at 60, which is pretty high...tuners like to keep it under 20 when tuning aftermarket ECU's
If its only happening in 4th or fifth gear, I'd say you're in the wrong gear to be accelerating that hard at the time, and putting too much load on the engine. if you were to drop back to 3rd and accelerate that hard, chances are you wouldn't get the knocking as it sounds like your car is fine the rest of the time....
Have you got an after market ECU and a tune? or still on the stock ECU?
Either way, its not good, so I wouldn't keep doing what you're doing...take it somewhere and get it checked out