As has already been mentioned, making windoze look like another OS is easy. Windowblinds and Litestep being but two. If you type in something along the lines of "windows shell themes" into google, you'll find loads of programs and themes. Make it look like BeOS, AmigaOS, OSX, OS9, Next or anything else you can think of.
At no point though did I say that simply looking like Amiga Workbench made Windows any better. We all know it's still relatively slow and bloated. I was just pointing out that OS4 may LOOK like a updated workbench... but it may also be big and slow too. Everybody is assuming we'll get something as lean and fast as the classic OS... but that is a dangerous assumption. We'll have to wait and see.
Besides, there is already a small, fast, memory efficient OS on PPC... look at QNX. I still doubt very much that a new OS running on standard PPC harware is going to make any great headway in to the PC/Mac markets.