The places I've seen so far (e.g. http://www.isni.org/ and http://isni.oclc.nl/xslt/DB=1.2/SET=2/TTL=1/CMD?ACT=SRCHA&IKT=8006&SRT=&TRM=foo ) display ISNIs as four groups of four digits. Since even the official site displays them that way, it would seem to be the expected way to do it.