Considering that the faces B1/B2/B3/B4 are employed only once, maybe you could divide the scans into 4 parts/scans w.r.t. time:
2nd half of 1st A + B1 + 1st half of 2nd A
2nd half of 2nd A + B2 + 1st half of 3rd A
2nd half of 3rd A + B3 + 1st half of 4th A
2nd half of 4th A + B4 + 1st half of 5th A
this way you would not high-pass-filter (HPF) the data, as the resulting time series would be short. In fMRI experiments the same stimulus is often applied many times, because SNR/CNR is low, and by increasing the length of the time series you increase the degrees of freedom. But you apply each stimulus only once, so I do not think you would lose any power making 4 short scans out of the long one.
Apart from the HPF issue, I would be maybe worried about the fixed ordering of the faces. This might be a confounder. However, this can depend on the objectives of the study and the differences between B1/B2/B3/B4.
Best,
Wiktor Olszowy
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