I have the 4.0 and there's very little difference between it and the 4.2 - on/off button and another FMQ setting, plus a few more stitches. In answer to the question about the straight stitch plate, the 4.2 doesn't automatically recognise the plate, but there is a simple setting in 'Tools' you can set, called 'Stitch Width Safety'. Even if you switch the machine off and switch on again, the reminder will come on, saving those pesky broken needles, lol.
I recently 'test drove' a Performance 5 at a quilt show and was very impressed. As well as the features previously mentioned, I really liked the 'Stitch Creator', where you make up your own stitches, or alter the ones provided e.g. if you want some petals off one of the flower stitches, you just 'erase' them! These can also be saved to the memory. There's also a monogram stitch and the screen is much easier to use - no endless pressing buttons and the stitch categories are arranged better.
Why haven't I bought this wonderful machine? I'm still dithering over the Bernina 750QE which I haven't properly tried. The Bernina sales person at the quilt show was rude, so I walked away. If that was his attitude before he got my money............