flooring joist

[euroslob]

Hello
i have a 400 yr old house without any listing, we have just taken down the plastered ceiliing one of the joist is well wood wormed i need to replace, ive measured for a new oak joist but one end measures 5in and the other measures 4in what would be the best way of dealing with it get a beam cut to the desired lenght or get a 4in and build it up at one end what does people think?

Thanks
Richard..........