UCS

A deprecated character-set, extending ASCII to support additional language characters.
Like with ASCII, integers are mapped 1:1 to characters.

UCS-2

ASCII maps the numbers 0-127 to latin characters (lowest 7-bits of an 8-bit integer).
UCS2 uses the same pattern, but uses numbers 0-65535 (a 16-bit/2-bytes integer).

UCS-4

ASCII maps the numbers 0-127 to latin characters (lowest 7-bits of an 8-bit integer).
UCS2 extended that table to use 16-bit/2-byte integers.
UCS4 extends that table to use 32-bit/4-byte integers.

UTF-32 apparently started as a subset of UCS-4, but grew to support it's full range of characters with additional utf semantics.