You can use something like this:

Note: DummyMVParam is a multi-value parameter in my report.

so this is what my test is doing:

If there are less than 4 values selected from the multi-value parameter, show the values in a comma-separated list, otherwise show "lots o values".

=IIF(Parameters!DummyMVParam.Count<4,JOIN(Parameters!DummyMVParam.Value,", "),"lots o values!")