Nope , If I were you, I'd stay where you are with the 2 year fixed of 3.99%. That’s a good rate considering the ECB will move again in Dec and more then likely again a few months into the New Year.
By Dec the best NIB rate you could have would be 4%.
Also if you break out of that fixed rate you'll be charged a penalty. Wait till your fixed rate is up and then see how the market is and who's offering what at that stage.
There will be loads of competing offers available soon from the other lending institutions.