First, you stamp slightly enhanced, to the right a genuine SG 4 that sold for $25,000
There is an obvious lack of detail in your stamp especially when you look at the background behind the Queen and the edge decorations.
Many lines & dots are missing in the country and value tablet
The color is too bright and not dull as it should be
Also it is in too good a shape for a 150 year old stamp without the edge blur inking of period stamps
SO, what is it?
Most of the forgeries of this stamp are rather crude and this is not.
Most good forgeries have a watermark impressed with oil & a die.
It is obviously very good and would fool a few people.
So we have some choices
An 1867 SG65a rose/carmine with trimmed perfs as this poor examples that tried to fake an SG4 ??
A cut out from the 1950 London Ex sheet
Or my preferred choice
A Peter Winter Forgery
Note the weak spot in the upper left frame which is the same as yours
Oneglia & Jeffries also made good forgeries
One Oneglia sold at a prestigious action as a genuine
My 2c worth without actually seeing the stamp for watermark & paper
Peter Winter made most of his forgeries in the 1980’s
His work may well be the best overall of any forger
Like these gems
He never sold his works as “forgeries” but accurate reproductions mainly under a company called House of Stamps
A “bonvivant”, opera singer and artist who reproduced many of the world classics.