When did Eames 4-star bases change?  

Chair bases
I'm referring to the chair bases, in case that's not clear. I've heard they were changed to comply with US safety standards. If true, what year did Herman Miller start producing the 5-star chair bases?

Aluminum Group?
Are you talking about the Aluminum Group chairs?

Yes
The Aluminum Group chairs.

also would like to know more...
also would like to know more about it. at some point they changed the foot right? from the "straight" design with rounded ends(similar to what the lounge chair still has as a foot), to the now, bended chrome shiny foot

Was just browsing
I was just browsing through the archives and stumbled onto this posting from a while back. I just looked this up a week ago so I thought I'd post what I learned. 🙂
First, the foot structure went from four tines to five some time in the mid 90s to comply with OSHA regulations. Basically a five star foot has a much harder time tipping over than a 4 tined foot.
Second - the original aluminum group foot - also called the contract base - was used from the inception of the Alu group in 1958 through the introduction of the Softpad group in 1968. For whatever reason the footing was retooled in 68 and became known as the Universal base. The name seems fitting since it has appeared on all of the aluminum group, softpad, & time life chairs since then, as well as the segmented base table, and much of the standing signage and display pieces used in the Eames' exhibition designs.
Personally, I like the cleaner, crisper look of the original contract base.