I always thought this referred to Portuguese colonial currency

According to this:

THE DIARY OF CAPTAIN NEIL DOUGLAS 79th FOOT, 1809 TO 1810
Antony Brett-James
Journal of the Society for Army Historical Research Vol. 41, No. 166 (JUNE, 1963), pp. 101-107

"The Portuguese dollar was worth 4s. 6d."

http://www.jstor.org/stable/44222492

*

Of unusual interest:

THE NINETEENTH CENTURY TOKEN COINAGE OF GREAT BRITAIN IRELAND THE CHANNEL ISLANDS AND THE ISLE OF MAN...
W. J DAVIS

"The Four Shilling pieces issued by the two public-spirited tradesmen of this City (Whitchurch & Dore) have been subjected to chemical analysis by Drs. Chichester and Wilkinson, and they are found to be equal in purity to Portuguese dollars, from which they are probably made, the proportion of alloy which they contain being in per cent, of copper. "