I can’t decide.
Get a 4th axis with 1:50 or 1:100 gear reduction and big chuck or
get a combination of 4th+5th axis with 1:8 and 1:1 reduction that may not have the holding torque required for real world machining but allows for smooth 5 axis surfaces when milling organic shapes. 
Get both. Test. Then send me the one you don’t like as much. 
That’s a tough decision.
But, what software would you use to make proper use of 4 or 5 axes…
I have written my own 5 axis CAM proof of concept.
https://code.google.com/p/simplemultiaxiscam/
Interresting, that would be nice. Because I havn’t yet discovered a 4+ axis CAM for personal use that was within economic reach…
@DeskProto does very simple “4 axis milling”.
Albeit only using 3 axis Y+Z+A and not taking into account that a toolpath-distance changes the nearer you get to the center of rotation.
It can use a B axis for indexing.
The Hobby-version is cheap and containd the multi-axis features.
I use it regularly with a 4th axis for cylindrical blanks.
You can even do that in Mach 3 by just changing pinout for the motors.
???
You can define the pinout used for the 4th stepper but why would you change an existing pinout?
You’d loose one of the other axis.
I mean that if your CAM does not produce 4 axis v code, then the poor man’s version is to replace x with a.
Doesn’t work.
360°=0°, distances and speeds changes with the distance to the center of rotation,
vertical features are not vertical on a circular axis.
True, and that’s why it’s the poor man’s version. There are some diy software converters to wrap flat gcode to axial, but I’m looking for the proper CAM software.