Burst speed will also deteriorate with increasing shutter times. 4 fps will be under favourable conditions with good light, short shutter times and low ISO. Higher ISO may add a little processing time. Also things like noise reduction settings may influence speed due to variations in processing time needed for each shot.
And if the camera re-focuses between shots, which it most likely does, AF speed will be a factor too. So favourable AF conditions and a lens capable of fast AF will be factors too.
When in doubt..... Press the shutter.
That and turn off IS.