[FIX] Be robust to 4D T1w images
[FIX] Be robust to 4D T1w images

Fixes #792

poldracklab/preprocessing-workflow,oesteban/fmriprep,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/preprocessing-workflow,poldracklab/fmriprep,poldracklab/fmriprep,oesteban/fmriprep,oesteban/preprocessing-workflow