Hello Jon,
Tiger wouldn't allow you to have accessed all of your memory, rather it was limited to around 2.7-3GB or so since it was a 32 bit operating system running Hauptwerk 32 bit. If you want to access more RAM then you would need either Leopard 10.5.6 or higher with the Hauptwerk 64 bit OS X version running on Intel 64 bit processors. Any polyphony issues you had wouldn't have been due only to RAM, but alsothe processor speed. If you load an instrument in the proper amount of RAM then it should perform well assuming you have set your polyphony limits properly. If you exceeded the amount of RAM that the operating system would allow then you would also get poor performance since the operating system would have paged the data to the hard drive when physical memory has been exceeded which will always give very poor results and make it seem that your polyphony suffers.
Be sure that you have a read through this thread:
viewtopic.php?f=1&t=4539
about the Mac OS X 64 bit version of Hauptwerk (which has of course since been released) to understand the memory allocations for OS X Leopard.
In general, we would recommend that you upgrade and to use the 64 bit version of Hauptwerk to allow more memory to be used and to achieve better performance. If you can install 8GB then that will certainly help, although the Skinner can be run successfully in 4GB with the proper memory saving settings. Be sure to read the memory requirements for the Skinner here:
http://www.milandigitalaudio.com/skinne ... ements.htm