Nonstandard round duct size?

[SarcasticDwarf]

The previous owner pulled a 6' section of round duct out of our basement that now needs to be replaced. What I am running into is the ductwork (Probably from the 1950's) seems to be about 4.25-4.375" diameter. Does this sound right to anyone? How would you go about filling in this section?

 

[fitter30]

Use 5" and either 5 x4 reducer and you might need a crimper to make a taper edge