I have the same issue, however, amp4ads-v0.js is not present on my pages. I viewed the source, searched for the javascript, and it's not there.
I have a theory. My site is a Google News source. Everything I publish is also converted into a Google News article. These are usually rendered as AMP on mobile devices. They're likely cached as AMP on Google's servers. I believe that when Page Speed Insights runs a test, it's also accessing these AMP versions, which contain the amp4ads-v0.js script. I can't view the source of these AMP pages, but they're definitely AMP. Even though a small fraction of my page views come from these AMP pages, I am getting dinged for it. I joined Google News, yet it appears to have done more harm than good for my site. I stuck with it, because not everyone is accepted into Google News. It was a huge honor, but now that the honeymoon is over, I see that it seems to hurt more than help, because it produces horrible Page Speed Insights tests, which Google uses as a (weak) ranking signal.
I may consider closing my Google News Publisher account. I just unpublished my site to see what happens. The reality is, Google News favors large publishers. My site gets hundreds of thousands of page views a month from organic sources, but only 339 Google News page views in 16 months, because they favor large, corporate publications (not necessarily authority). I've been on Google News for two years and it hasn't helped one iota. In fact, it seems to detract from my overall traffic because it makes my site seem like a poor performer, thanks to those "super fast" AMP pages. AMP is ugly, slow and one of the worst technologies I've ever used. Google kind of forced people to use it, but now they're walking it back. Even the stock 2021 WordPress theme produces better mobile content than AMP.
My experience does provide some insight as to what Google considers an authority. It's all about favoring large corporate news outlets. Google News and Google Search prefer click-bait from Forbes over an article written by a top school grad with 20 years of software engineering experience. Their algorithms have no way of intelligently deciding which article is better than another. Forbes, in particular, thrives on fear mongering, click bait and misinformation, yet are the darlings of Google News. It's all about size and corporate status.
There's a point to my criticism of Google News. It's not for small publishers. If you're racking your brain as to why you see this amp4ads-v0.js script pop up in Page Speed Insight reports, yet you don't have it, it may be because you're on Google News. If so, you need to decide whether it's worth it. Look at the reports in Search Console. It's likely you only get a few page views a month on Google News, to the detriment of your actual site.
BTW, has anyone tried running Page Speed Insights on Google's web properties? YouTube.com gets a 17! Google Search gets scores in the 20s. The cobbler's children wear no shoes... Google tells us to do all of these things that they don't do themselves. Most established sites, like Bloomberg, The Verge, etc. have abysmal Page Speed Insight scores. The moral of the story is -- don't obsess over Page Speed Insights and Core Web Vitals, because they're not that important. It's a slight advantage. If you're spending more time fiddling with page speed than writing content, your website will suffer. The best way to improve page speed is to go with a new WordPress theme. The stock 2021 theme is excellent!
