I've been having a look through my textbooks. The simplest explanation of the metallurgy that I can find is from "Metals in the Service of Man" by Alexander and Street, a great overview of metallurgy for the general reader. I've appended a PDF of the relevant section - easy to do as my paperback copy is so well used that the pages are all falling out and therefore can be easily scanned
My interpretation is that, according to Tjark's useful data, LM4 has sufficient copper, silicon and magnesium to allow it to be solution treated. As cast it will cool slowly to room temperature. Over time it will age harden naturally. The rate at which this happens can be accelerated by heating to 170C. If the casting is heated to 500C, quenched and then heated to 170C the result will be 25% harder and therefore easier to machine.
Jo, my little furnace
http://www.modelenginemaker.com/index.php?topic=3483.0 can do this if you would like me to have a go....
Rod