I have the exact same response - I sometimes lose my place on my NST but it's more comfortable or easy or intuitive or something to turn the pages on the K4. On the other hand, as I'm reading I get very used to pushing the button to turn the pages on the K4, so that isn't such a big deal, and I only occasionally lose my spot with the NST. Also, I think my cover is touching the screen and that's what's doing it, so I'm trying to keep the cover open until the dead authors come up again to see if that helps.
I like the both and am glad I have both as I can easily use both stores, and my husband and I can each use one. I'm not sure which I'd choose. I think the Amazon store is a bit easier to use - at least on an iPad - so I might choose the Kindle based on that. (I got the NST - my first ereader - as a gift.)