The only safe way may be to look what's already in the system.
(Everything else than red color means .4 IMHO).

For Cleveland/Parker brake calipers delivered by Tost to non-
Schleicher manufacturers: they are ex-factory equipped with o-rings
for mineral/"Aeroshell" oil and "converted" to .4 (= o-rings
exchanged). These calipers receive a big red (removable) label that
says DOT4 and get a little "O" stamped to the outside (diam. ~ 5mm,
permanent) which may be hidden by other hardware when the caliper is
mounted to the glider.