They have been revealed in new images downlinked from the Nasa spacecraft, which is still some 90 million km from reaching the distant world.
Hydra and Nix were seen previously, but New Horizons can also now resolve Kerberos and Styx.
Pluto's fifth known moon, Charon, is much larger and brighter.
By comparison, it is relatively easy to detect, although in the new image-set just released it is burnt out with Pluto at the centre of the frame.
Pluto has a diameter of about 2,300km; Charon is half that.  Hydra may reach over 100km.
But the girths of the other three moons are probably just a few tens of km at most, and getting their faint forms to appear in the pictures from New Horizons' Long-Range Reconnaissance Imager (Lorri) means over-exposing Pluto and Charon.
The probe is due to make its flyby on 14 July.  Launched in 2006, the spacecraft has crossed some five billion km of space.
It is travelling so fast that it will not actually be able to get into orbit around Pluto when it arrives, and will have to grab as much data as possible as it races past.
The rendezvous will complete the reconnaissance of the "classical nine" planets of our Solar System. New Horizon's flyby will mean all have been visited at least once by a space probe.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
The New Horizons probe, heading for its historic flyby of Pluto in July, has now caught sight of all the known faint moons of the dwarf planet.