Are you referencing to client side, right ?
That is if my clients have set 4 ntp servers and one of them become mad, It’s possible to keep right time by the three survived servers.
My idea is to destroy Master server and to expose NTP[1-3] server to public network to retrieve time directly from stratum 1 servers of Internet. At this time I would add the fourth server.
What do you want to say with “multivalued DNS record” ?
In every client I will set n.4 different ntp servers which have different names e.g. ntp1, ntp2, ntp3, ntp4