To replicate click the 'import' link on top of any GridField in SS4-beta1.
Shows error message:
[Emergency] Uncaught InvalidArgumentException: Can't handle action "import"
Possibly same issue https://github.com/silverstripe/silverstripe-admin/issues/168
The CSV import is flat out broken for me in beta2 and Chrome - clicking it in admin/security or a ModelAdmin doesn't do anything, and seems to capture clicks afterwards - so the whole UI becomes unresponsive until you refresh the page. Raising priority.
FYI, there's a new design for this (modal based), see https://github.com/silverstripe/silverstripe-framework/issues/6431. For now the aim is to get the baseline working again.