Not sure why people think that is strange. The four year bit is not really a commitment to play for us for four years, it's financial security for player (career threatening injury, long term salary guarantee) and club (fee inflation).
Suarez, Ronaldo and I think Beckham all signed extentions to already long contracts that were blatantly "one more year" gentleman's agreements before being allowed to go, so I have no doubt the practice is very common.
After everything that has happened it's amazing anyone still looks at a footballer's contract and thinks "well, that's them playing for us in 2017 for sure".