We present a combined experimental and theoretical study of twisted double
bilayer graphene with twist angles between 1{\deg} and 1.35{\deg}. Consistent
with moir\'e band structure calculations, we observe insulators at integer
moir\'e band fillings one and three, but not two. An applied transverse
electric field separates the first moir\'e conduction band from neighbouring
bands, and favors the appearance of correlated insulators at 1/4, 1/2, and 3/4
band filling. Insulating states at 1/4 and 3/4 band filling emerge only in a
parallel magnetic field (B_{||}), whereas the resistance at half band filling
is weakly dependent on B_{||}. These findings suggest that correlated
insulators are favored when a moir\'e flat band is spectrally isolated, with
spin polarization at 1/4 and 3/4 band filling and valley polarization at 1/2
band filling.Comment: 5 pages, 4 figures, includes supplementary materia
