[happycube]

That was a totally different DOS 4, which only surfaced relatively recently. (And the shipped versions apparently weren't fully featured either)

[pjmlp]

The shipped 4 versions were a consequence of the project's failure.

Like almost everyone else I kept to 3.3, totally ignoring 4.

Afterwards I actually ended up using DR-DOS 5 for a while, until MS-DOS 6 came out.