You had to pick one from that set first? Kidding! I'm just not that familiar with the perforated issues, as opposed to the earlier imperf ones. But I'll give it a try.
First of all, I will say that most of the 19th c. Spanish stamps have been forged in one form or another. Some of them with dozens of different forgeries. Many of the forgeries from this particular set (1873) were actually made from stolen cliches.
I am not aware of any postal forgeries of the 4P, only philatelic forgeries.
Like most of the early Spanish issues, the engraver incorporated some small, hidden details in the design, to help minimize forgeries.
For the 1873 issues, there are 2 such marks that are "generally" referred to as "secret marks"

#1 points to the area where the horizontal background lines meet the forehead. Actually, on genuine stamps these background lines fall just short of touching the forehead. Some stamps are printed a little heavier and may appear to be touching, which complicates things a bit more.
#2 points to the small triangular area between the palm of the hand and the branch. The smallest background line has a small break in it on the left hand side.
These can be used for all the values in the set. Unfortunately for your 4P, it appears that portions of the cancellation are covering these 2 key areas.
But, there are some other things I would consider "red flags".
A) The cancellation is not a known cancellation for this issue.
B) The color appears to be more orange than brown. but this could be just my perception on my monitor.
C) The 2 areas circled have me puzzled. the background lines look like they may have been hand drawn in, in the large oval. Also, the background lines in the small circle appear to be wavy. Is this a anomaly from the scan?

One thing to remember is that these high values are commonly found with a telegraph punch. It is also very common for forgers to try to "mend" the telegraph punch and hide it with something on the back. It is also tempting to try to make a patch for the telegraph hole and make an attempt to draw in the missing details.