According to the manual page, you can use
--include with either the index or the shortname for the given entropy source specified in
-l to enable it. In your case, you would run the following:
rngd -n 4
However, in the same manual page, there is a warning about enabling this source:
Entropy gathered from the NIST network entropy beacon. Note that this entropy source is disabled by default as it should never be used for cryptographic purposes, or any use case in which random data should be known only by a single entity.
The problem is that the NIST entropy beacon sends the random data to everyone over the network. It is not intended to be used for secret keys (the ouput isn't secret), but for getting unpredictable randomness. In fact, it allows anyone to see a list of previous random outputs! From their FAQ:
The Beacon is designed to provide unpredictability, autonomy, and consistency. Unpredictability means that users cannot algorithmically predict bits before they are made available by the source. Autonomy means that the source is resistant to attempts by outside parties to alter the distribution of the random bits. Consistency means that a set of users can access the source in such a way that they are confident they all receive the same random string.
Bold emphasis mine. This means that everyone gets the same output, so it is not secret after it has been released! In fact, the beacon only releases a new 512-bit output to everyone every 60 seconds, and the previous output is stored and available for anyone to see. Now, while rng-tools will only mix the data with the random pool in order to prevent any sources from reducing security, using the NIST entropy beacon to supplement your kernel random pool is unnecessary and won't really provide you with any cryptographic benefits, anymore than mixing in the previous winning lottery numbers would.