Ok well like Silyavski says ideally you need a milling machine. BUT because of your other need to mill aluminium it's actually not that straight forward.?

First to do the valves you will 99% need to grind them. This means you'll need some form of Grinding attachment to bolt to the Spindle column. The Forth axis will need to be quite ridged well setup and with good resolution. It will also need to be able to handle coolant which you will definatley need for grinding.

Now we get to one of the Less than ideals.? Milling aluminium ideally requires high spindle speeds and most common milling machines don't have the spindle speed for cutting aluminium correctly or with any decent feed rates.

The other less than ideal is the fact grinding is very messy and abrasive which plays havoc with milling machine slides and Ways. It basicly mixes with coolant and turns into grinding paste wearing the slides etc away in short order. At best this means very careful clean up before doing any other milling with some serious guards for coolant and at worst regular strip downs to properly clean.

Personally if I had your needs then I'd split the work between 2 purpose built machines.!!. .

First a Stiff fixed Gantry machine designed to mill aluminium correctly.

Then a purpose built 4th axis with grinder attachment to soley grind the valves which can handle the mess with sealed protected linearings etc.

One machine like a milling to do both will suffer badly IMO and be less than ideal at both.!!