You've confused the terms series and parallel. Normally the 4 200v units run in series giving a total of 800v. But for charging they can do a 2S2P arrangement (2 units in series which are in parallel with another 2 units in series) to give 400v.
Series is end to end; add the voltages. Parallel is side-by-side; add the capacities.
To achieve fast Cybertruck charging on the current non-800V infrastructure, Tesla splits the battery in packs, just like phone makers do. Tesla promises 128 miles of added range for 15 minutes of Cybertruck charging which beats the Model Y charging speeds.