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If you're in doubt, it can be load tested. My 135i battery* went for about 11 years and I replaced it only because I felt like it must finally be time, even though it still seemed to be working like new. 4.5 years won't typically be an issue, it could be of course, but if so it would be on the early end of fail times.
* Assuming you're talking about the trunk mounted AGM main battery for starting the car and such. If so they are all basically the same, if you're wondering why I'm bringing up a 135i (what I have owned for 13 years and will be replacing if I manage to get a Z4 allocation someday).
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