Were you looking for a mirror finish from face milling, or on a 3d surface? Are we talking about a CNC knee mill? You have plenty of rpm to get a good result from a face mill in alu, but that would take some time on a 3D surface.
For the best finish on Alu with a face mill I like the 45° insert type - this is the one I use: Maritool 45° face mill I also use this face mill (with different inserts) in steel, with a near mirror finish too - take a look at the video on that page, I'm getting comparable results on my (manual) knee mill. Take a light finish cut after your roughing cuts - knee mills are comparatively flexible, and you won't get a brilliant finish with a heavy cut.
If you need to mill up to a shoulder, I find this one gives nearly as good a finish: Maritool 90° face mill
In both cases you'll get the best results with the inserts designed especially for aluminium (usually very sharp, polished inserts).
You can get a nice finish with on a flat surface with a decent sharp end mill, but generally the swirl marks will mean you wouldn't consider it 'mirror' finish. On a 3D surface more RPM would greatly speed things up, but generally you'll want a very small stepover and stepdown, and your CAM software will need special 3D strategies to get the best results. I get good results with a 1/4" EM @ 25,000 - 35,000 rpm and feed rates around 100 ipm. With a 4200 rpm max, just drop the feed back proportionately and keep the coolant on - either flood or an air/mist blast. The important thing is to keep the chips out, and the cutter cool.
You can also often add a VFD to get more speed from your standard spindle motor - several others are doing this with good results, some are getting as much as 8500 rpm that way. I don't know what effect this might have on the longevity of your spindle bearings though. I use a VFD on mine, but haven't needed to turn it up past 4200 yet.
Hope that helps