Bank of Upper Canada Red Protector

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This one is a Bank of Upper Canada Red Protector $4 dollar one pound note.
Dated Dec 1st 1846, #19 in the series. There are 7 others know all in the
catalog dated from 1851-1856. This one is not in the catalog and the only one
dated in the 1840's, also a low serial number 10019 which is #19.
What could a banknote like this really be worth to a true collector knowing there
is only one.

Notes of the red protector issue are so rare that they are collected only by type, not by date.  (That is the case for any manuscript dated rare note; date collecting is popular for notes where the surviving population is sufficient to support it, such as the first issue Westmorlands)
There are more of the $4 denomination known than all the other denominations combined, ie. 1x1846, 2x1851, 2x1852, 3x1856.
It is known from documentary evidence that the first printing orders for this series began in 1843, but the first $4 sheets were not ordered until April 1844.
Surmising the condition of the note from the image of the face in this thread, I would suggest a fair and realistic market value would be something of the order of $4,500, perhaps a little more.  This is well in advance of the present catalogue value, and is based on recent sales of Bank of UC notes in general.  The 1846 date is not a factor; type and condition will determine the price. 
Incidentally, what is the evidence that numbering started at 10000 for this issue?  Numbering was sequential from beginning to end of the issue, and since it is pretty clear that $4 notes were issued earlier than 1846, are we to believe that fewer than 20 notes were issued before Dec. 1846?

The reason I believe the serial # of this one is #19 is because it is noted that these were ordered sometime in 1844, so they would have started to be used sometime or late in the year of 1844.  the two that are know from 1851 have the serial numbers 18137 which was dated Nov 1st and 18672. So if it took 5 years to go from serial # 10019 to 18672  which is only 8000 notes how would the first 2 years produce 10,000 notes. So being logical I conclude that the serial numbers started at #10001. Yes they are very rare and catalogue price means nothing, besides this one is not even in a catalogue. The value of it is what a collector that wants to own a Red Protector is willing to pay being there are no others to be found.