Understand. Size problem will be checking the 4" flow (add the open ended pipe end) aginst your needed flow capacity for the test.
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I was (recently) very uncomfortable 50 ft below and 150 ft (horizontal) from a 1.5" line blowing down a MS line - wearing ear muffs - and could only talk to the guy beside me by shouting. We made no decible check when the blowdown started - just moved away.
Further back - around the corner and towards the parking lot - it was manageable to to talk to each other. (The corner of the building probably made most of the difference.)
Size of plume was minor - not really noticeable nor memorable, maybe because of the particular weather conditions that day. As a "renewale energy capacity check" I'd think you could even advertise the test to avoid public comment and deflect criticism. Also, as a "test" the public tends to accept air raid sirens and tornado warnings with no protest. As regular operating procedures - then they get upset.
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Thoughts: Blow it (the plume) up - not sideways or down. Condensing steam is more noticed, but noise is reduced - and the noise is directed upwards, not reflected or blown sideways.
If you are in the boonies - probably not as much problem than if you had nearby (non-industrial) neighbors.
Length of time: Shorter scope (1/2 hour or less) will attract less attention, be more accepted than a several hour blowdown test. But - if as I suspect you need a capacity check lasting a long time, night time will be much more noticed than daytime - even if no residents are closer than 3/4 mile. So start your test as soon as possible after people are up and awake - not daybreak.
Double pipe (relief outlet inside a temporary larger pipe) will reduce noise as well - but not be a permanent official (and expensive) muffler
