Distribution of BH spin for primary BHs in grey, for remnant BHs in blue, for the global main BH population in orange, and for the global population weighted by the M_* distribution of BH merger hosts, in red. All distributions correspond to redshift 3.5 < z < 4. Pre-merger primaries have higher spins with respect to the global BH population but similar spins to an M_*-matched sample. Post-merger BHs have typically lower spin than prior to the merger, signifying that there is no full alignment. Post-merger BHs with negative spins generally result from spin flips at the merger.