3D Scatterplot of 4D data

Hi all,

A little help in plotting 4D data. I have a file with x,y,z,v data.

For each x,y values there are different z & v values: The data is from  
CPT testing where there is a resistance measure taken at the tip, at  
even intervals, as the probe is driven into the ground.

x1,y1,z1,v1
x1,y1,z2,v2
.....

what I would like to do is plot a volume plot of the x,y,z data in  
different colours, where the colours are based on the values of v.

Thank you in advance.

Regards

Domenico