from my own experience, you would be looking at a scanner only solution at the moment as buying an in-house mill for a 4 man lab would take to long to pay off.
lly you need to figure on a 3-5 year ROI.
so i would say a scanner to out source that also has the possibility to run an in-house mill at a later date.
Its really hard to know what to go for at the moment as things are moving so quickly that what you buy one day, could easily be out of date the next, if you buy in to the wrong system.
3shape seems to be leading the charge at the moment and their older scanners are still supported in the latest software which shows their commitment to their user base. 2010 update finally has digital articulation. IF it works this will be one of the holy grails CAD/CAM has been waiting for.
An open system is a must, do not get tied into one manufacturer as they will shaft you in material costs or some other way.
as far as in-house milling, a dry+wet mill is the way to go now i think, so you have the option of zirconia and glass, although there are still limited options in this respect for smaller labs.
dont belive most of the marketting/spec blurb on any system as most info is manipulated to give results that you will never acheive in day to day use, such as accuracy and output volume.
my 2 cents