Not that surprising. If they were planning on doing a surprise name change at the end of April, registering the trademark a month early would have spoiled the surprise. Plus it wouldn't give any advantage. Voltswagen would be protected by their Volkswagen trademark, and trademarks don't even offer protection until you submit proof of you already using them in commerce. And you don't have to register trademarks to have common law protections in the US, and they aren't planning to use it in Germany.

This is the best evidence yet that this is indeed an April Fool's prank.

I'm not certain but if they were serious there would be legal filings of some sort. I'm not an expert in trademarks and I understand that sometimes trademarks can be filed but held in confidence until a company is ready--Apple does this if I recall correctly.

Whois is also less than certain. Domain redirects to a generic parking site.

https://www.whois.com/whois/voltswagen.com

(IAAL but this is not legal advice, and I could be incorrect in certain aspects. Consult a licensed attorney for legal advice.)

With respect to the USPTO, I believe only patent applications can be filed confidentially. FCC applications can also be filed confidentially. But I believe trademark applications are published immediately upon filing, because the essence of a trademark is use in commerce. You don't need to file a trademark application with the USPTO to be protected by trademark law, but it is an important element of notice, which is relevant when determining certain aspects of infringement claims.

If it was, it's gotta be one of the good ones. Bordering on truth (still people are trying to work out whether it's real or prank), good media coverage, and even if called out – will only end up having a positive association with their electrification efforts.

Taking a look at the website the ID.4 is branded as "Voltswagen ID.4". Looks pretty serious to me.

https://www.vw.com/en/models/id-4.html