Reducing the energy bandwidth of electrons in a lattice belowthe long-range Coulomb interaction energy promotes correlation effects. Moire superlattices-which are created by stacking van der Waals heterostructures with a controlled twist angle(1-3)-enable the engineering of electron band structure. Exotic quantum phases can emerge in an engineered moire flat band. The recent discovery of correlated insulator states, superconductivity and the quantum anomalous Hall effect in the flat band of magic-angle twisted bilayergraphene(4-8) has sparked the exploration of correlated electron states in other moire systems(9-11). The electronic properties of van der Waals moire superlattices can further be tuned by adjusting the interlayer coupling' orthe band structure of constituent layers(9). Here, using van der Waals heterostructures oftwisted double bilayergraphene (TDBG), we demonstrate a flat electron band that istunable by perpendicular electric fields in a range oftwist angles. Similarly to magic-angle twisted bilayer graphene, TDBG shows energygaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulator states. We find that the gaps of these insulator states increase with in-plane magnetic field, suggesting a ferromagnetic order. On doping the half-filled insulator, a sudden drop in resistivity is observed with decreasing temperature. This critical behaviour is confined to a small area in the density-electric-field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. The discovery of spin-polarized correlated states in electric-field-tunable TDBG provides a new route to engineering interaction-driven quantum phases.