If acceleration is constant, then it's the same as the average acceleration over the 4 second interval:
[tex]a_{\rm avg}=\dfrac{\Delta v}{\Delta t}=\dfrac{22\,\frac{\rm m}{\rm s}-12\,\frac{\rm m}{\rm s}}{4\,\rm s}=2.5\,\dfrac{\rm m}{\mathrm s^2}[/tex]
