The 4 Trangka, blue color stamp from the 1914 issue of Tibet is the rarest and most valuable stamp of Tibet.

GENERAL RULE:
4 Trangka, The small rays from the ring in every corner of the stamp point inwards

8 Trangka, The small rays from the ring in every corner of the stamp point outwards

Unfortunately, ebay image server shrinks the images dramatically but I feel this guide will, help you and eventually safe you a lot on money.

Being able to distinguish between the two values of the second series helps in identifying many 4 Trangka forgeries. Many stamps being offered as the 4 Trangka value are in fact forgeries of the 8 Trangka, but printed in blue.

Of course, this rule is not valid for all of the 4 Trangka blue forgeries in existence but valid for at least 95 % of the forgeries offered.
These stamps are frequently offered on cover, any cover with these stamps (and other Tibet stamps) looking like the ones shown here are ALWAYS FAKE