Hello Stuart Cotts,
It seems we have a problem with the WPF assemblies in Blend 4.
Until we find a solution you can use the following workaround - go to the Assets tab and find the GridViewRow asset. Add it to your page. Now if you go to Object > Edit Style > Edit a Copy it will be available for you and you can edit the row style. When ready, simply remove the GridViewRow you have added and apply the modified style to the gridview.
Hope this helps.
All the best,
the Telerik team
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