I noticed a couple of things, especially in IE8. I haven't dug too deeply into the code yet, so this is mostly from the perspective of someone just looking at the pages.
On the example at the bottom of the page the tooltips on the buttons look really bad in IE8. They render way too wide for the text they contain. So, instead of seeing 'Add new row', it's like you see 'Add new row' and then a whole bunch of empty space to the right. The same tooltips look much better in FF, though there is a lot of vertical space. Is it possible to style the tooltips like they used to be in 3? I'm just not sure I like the color now compared to the default grid color scheme.
IE8 gets the opacity of the icon on the Remove Something button wrong. The text still looks disabled, but the red circle icon definitely does not. In FF it looks ok.
None of the fields busted out the '...' for the overflow - now they just look cutoff. Is this intentional?
In IE8 there is an egregious amount of vertical space between rows on the first row expander example. The rows are noticeably taller than every other example on the page. Again, in FF it looks much better.
I kind of liked the way the grids in 3 had a different background style for the plugin column. I guess this is a matter of taste. Will this be easily configurable in 4? Sorry if this is obvious; like I said, haven't had much time to look at the code yet.