How long does it take for DNA to degrade Can i still use 3-4 years old bacterial DNA template for my PCR? They were kept in -20 all this while. Will the DNA degrade?

Degradation of DNA is gradual, but continuous, even in the freezer. However, they can sequence DNA from ancient animals in permafrost, or Neanderthal teeth. Fresh is always best, but there is no reason not to try PCR on your samples.