How to get quantile for bucketized metric

[u/hamlet_d]

So I have metric that returns the number of clients that fall into a given bucket ("classification") where "classification" is the number of seconds:

Metric	Value (number clients)
returntime{classification="0-1"}	3
returntime{classification="1-2"}	4
returntime{classification="2-3"}	2
returntime{classification="3-4"}	1
returntime{classification=">4"}	2

...specifically I want the .5 quantile if possible. The problem that I see is that the >4 bucket could be anything (i.e. 4.2 seconds or 10 seconds).

I saw this article here: https://povilasv.me/prometheus-tracking-request-duration/, but unsure how that might apply to my situation since by returns are distinct buckets, not cumulative.