> UTF-32 encoding form: The Unicode encoding form that assigns each Unicode scalar value to a single unsigned 32-bit code unit with the same numeric value as the Unicode scalar value.
UCS-4 is defined in ISO 10646. Before the 2011 revision of the standard UCS-4 had a 31-bit codespace, since then the definitions of the ISO and Unicode standards have converged and UCS-4 and UTF-32 are now synonymous.
> while UCS-4 currently encodes all of Unicode, there's no guarantee it will "forever".
There is. Unicode has a codespace of U+0000..U+10FFFF. There are 2^16 + 2^20 − 2^11 assignable codepoints, in Unicode 15 a total of 149,186 (13.4%) have been assigned.
Even if the Unicode codespace were to ever be extended again (it won't), the only encoding that would become incompatible is UTF-16. In fact, both UTF-8 and UTF-32 are trivially extensible and used to be wider encodings, but were restricted to 0x10FFFF arbitrarily to match UTF-16 limitations.
EDIT: My wording is very ambiguous. There are actually 286,719 (25.8%) assigned codepoints in Unicode 15: https://www.unicode.org/versions/stats/charcountv15_0.html. Still, the point stands.