Ryanair has been able to meet this 4 aircraft shortfall during October, November and December by extending the flying programme on a number of older 737-200 series aircraft due for retirement.
I hate to cast doubt on Ryanair's maths but 4 aircraft delayed one month is not 4 aircraft short for 4 months, it's 1 aircraft short for 4 months. Seems like the same maths that promises flight for 99p.