4th bedroom or master bath

[Dave Charron]

So I just picked up a 4 br 1.5 bath Colonial built in 1930.  2133 sq ft.  The 4 bedrooms are on the second floor along with the full bath.  Its got a large den on the first floor that could be another bedroom.    The house is overall in solid condition, just needs an intense cosmetic rehab with new kitchen and baths. 

I am contemplating whether it is a good investment to turn the 4th bedroom in to a master bath instead? I would probably spend $5-6K. ARV for this house will probably be $135 to $150K

Any input?