Last four centres

Centres are built from bars, two centre pieces side by side, joined into a full centre with an inner-layer turn and moved out of the way with an outer turn. Start with two opposite centres, then build the side centres around them. Because the centres are not fixed, they must be placed in the colour scheme: opposite colours opposite, and the side colours in the order the corners need. The last two centres share their pieces; a few algorithms handle the patterns that are slow to see.