Using Sharepoint Foundation 2010 I've edited v4.master, added a ref to a new CSS file, saved the changes and they are applied to the home site no problem.
However when I create a subsite it uses the old version of v4.master for some irritating reason. I'm vaguely aware that some of the publishing features from SPS are not included in SPF so lots of the online advice isn't relevent (?)
If there is a way to make new subsites inherit the new V4.master I'd be very grateful.
This is only possible with a custom WSP or an HttpModule, both requiring .NET code. The Server versions of SharePoint handle this easily with the Masterpage option, a quick google search for "foundation masterpage solution" gave good results, including: