The Seiko caliber 4RXX family of automatic movements is basically Seiko’s branded versions of the SII NH family. These are the movements that Seiko uses in their own watches. The NHXX equivalents are sold to other brands.
List of calibers in the Seiko 4R family
Movements without hacking, handwinding functionality:
Seiko 4R calibers cannot be purchased, if you want to replace the movement in your 4RXX powered timepiece, then you will need to send your watch to Seiko Service, or purchase an NHXX movement to swap it out.
Seiko 4R to NH Conversion
The list below gives a general idea as to the crossover caliber numbers between the 4R and NH series. Keep in mind that while the watch community mostly goes on the premise that these calibers are interchangeable, there may be slight differences between 4R and NH calibers. The caliber numbers are easy to decipher, mostly just switch 4R with NH.