A cylinder was used to make watermarks on these values.
this cylinder had enough crowns and letters on it to make 8 sheets of paper in one full round.
naturally at this age in time, nothing was perfect, so small variations on the letters and crowns ensures us (who work with these) to identify these 8 different sets of crowns and watermarks.
small inaccuracies in placing the stamp sheets means there is a little margin of error in this though.. where it gets hairy is that these sheets could also be placed inverted (inverted watermarks) and mirrored both normal and inverted watermark.
the 4 øre came in 130 prints of double sheets (200 different positions) add that each position could be 1 of 8 sets of crowns and letters, and then times that by 4 and you'll have 832.000 different 4 øres... all in theory of course as some prints won't have inverted watermarks or mirrored as all.
that was a little more than you asked for.
here's a picture of the watermarks

you can quite certainly place your stamp as position 50 in A or B sheet, and from the date and the inverted frame you can place your stamp in print 121, which is the latest print that has inverted frames with watermark 112.
and this matches the general 2 month rule(see later on)
with access to literature, we can see that B50 has frametype 5A, which your stamp does not, so your stamp is print 121 position A50.
Hy-Brasil is correct in his approach, his source of information is inaccurate though (which most foreign literature is) the first print with a watermark 113, is print 125, which was delivered for control between Dec. 30th 1903 and Feb. 13th 1904, a general rule of thumb is that you won't see a a certain print until around 2 months after the first date of delivery, because there was extensive control, and it took time to deliver and use left over supplies and so on.
so you're unlikely to see a watermark 113 on 4 øre until late january. 04.
here are 3 examples of watermark 113, all unused to give an idea of varying color.

another aspect was the inverted frame, which watermark 113 has in print 127, position 91-100 in both A and B sheet.
you're likely to see this offered for sale incorrectly, here are 2 tell tales to identify a correct one.

firstly, it must be cancelled late 1904 from august or later preferably.
secondly, most bi colored stamps from position 91-100 will have the bottom frame and oval smeared. this is true for this print at least.
look at the oval under "FRIM".
the easiest is of course to find a vertical pair like mine with watermark 113 and you're 100% certain, but these are kinda rare, and mine has inverted watermark as well for extra "loveliness"
hope this helps, and maybe give a little more interest in the world of collecting bi colored.