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You may get no traffic from ranking #4 these days, especially on queries with a competitive paid portion of the SERP. What I would do is stop assessing the "what if" scenario's and start focusing all your energy towards acquiring those editorial type links grasshopper was talking about.. right now! You'll get that ranking and secure it for long-term traffic.

Single-page penalties are uncommon but possible. Is the page indexed? That's step one. If it is, are you seeing duplicates when you search your index (use the "site:" operator) for the page's title? If so, it could be a filter. If you're indexed but not ranking even on exact-match terms (in quotes), it could be a penalty.

The most likeey cause is over optimization or similar page content. If you scroll through the serps for the same keyword that would usually bring up the disappeared page and another of your site pages comes up further down like page 3, 4 or whatever, you can be pretty sure one of those two issues is the cause.

Thanks Ade, I will check this company out.

Martijn is 100% correct so please listen to him. Thumbs up.