Why does Q4 results (31st March) being announced in June affect the stock as it is 2 months old and other events might have overtaken the impact of the result?

Yes, other events might have overtaken the 4th quarter results, but the financials of a company, including older ones, does give you a picture of how that company is doing, how it is run, etc. Companies have to look at what they think might impact things following the date of their financials - it's part of disclosure rules.

Your question is looking at a company from a short-sighted point of view, to only see what has happened in the last month or two. A lot of investors do tend to look at things in this manner, but they shouldn't. They should be looking at the overall picture. How the company is being handled over a long period can have a huge impact on how things might be handled in the future.

In the end, it's really just a crap shoot of what MIGHT happen in the future, which no one knows. So having a good history of the company and how management handles things gives you a better overall view of the company. You have to look past the end of your nose.