Note that CD4 is very strongly down-regulated by any activation that
engages PKC (i.e., most mitogenic)--don't use it to identify cells!
You can also identify T cells with CD5 (including activated).
Also you could try CD38. Skip Maino at BD has recently published an
article about measuring activation ex vivo using CD69. Upregulated
2-12 hours after signal is received.
mr