I've tried to find answer to my question but no luck. I was in this country on H4 for four years and changed to F1 in 2005. Is substantial presence test regardless of any VISA previously and count only against the number years on F1? I would appreciate any help. Thanks.

For me you are resident (for tax purposes) during years of H4. You will remain resident if you keep on living in the U.S. and it does not matter if you have H4 or F1.

Let us see what AtlantaTaxExpert has to say.

Actually, the previous years on H-4 does NOT affect your F-1 status. The day you swicth over to F-1 is like you are just stepping off the plane from your home country.

So 2008 will be considered your THIRD year of exempt status under the F-1 visa.

Your exempt status will expire on 31 December 2010 if you do not convert to a different visa earlier.

I am not sure about this: You were resident (for tax purpose). You are still living in the U.S. and you become non-resident?