The amsmath package provides also a subequations environment to make it easy to number equations in a particular group with a subordinate numbering scheme. For example
\begin{subequations}
...
\end{subequations}
causes all numbered equations within that part of the document to be
numbered (4.9a) (4.9b) (4.9c) ..., if the preceding numbered
equation was (4.8). A label command immediately after
\begin{subequations} will produce a ref of the parent
number 4.9, not 4.9a. The counters used by the subequations
environment are parentequation and equation and
addtocounter, setcounter, value, etc., can be applied
as usual to those counter names. To get anything other than lowercase
letters for the subordinate numbers, use standard / methods for
changing numbering style [5, .3, § C.8.4,]. For example,
redefining theequation as follows will produce roman numerals.
\begin{subequations}
\renewcommand{\theequation}{\theparentequation \roman{equation}}
...