CRM 4 IE compatibility

I have a customer who is concerned that the latest version of IE that they are using is not compatible with CRM 4.0 as I believe that only works with IE 6,7,8 and 9 and does not work correctly with IE 10 or 11.

I presume their only options are to:

1, Stay on a IE version which is compatible with CRM 4.0

2, Upgrade CRM to 2011 then 2013  which I will be discussing with them considering their existing environment and work that may involve.

Does anyone have any alternative suggestions they could consider as a workaround?

Thanks.