Usually, you need to support the browsers listed in your spec.
Where you genuinely have free rein, I suggest that you needn't support further back than Gen6/c.2001 unless you need to demonstrate your ability to do so (i.e. your personal site as a web dev /your corporate site as a web dev shop). Gen 4 is considerably tougher than the others, so you'll have to consider the cost/benefit for yourself.
Fwiw, virtualisation tools are a great way to do x-browser testing, but a respectable guerilla option exists in Multiple IE (IE being the only real problem in this regard, as per...)
Browser Timeline fwiw