[2021] Anyway to find/buy the BMD-4?

Hello!

The BMD-4 is available during the current in game event. Summer Landing: https://warthunder.com/en/news/7267-event-summer-landing-en

During which, you can complete daily tanker missions to earn Stars towards the BMD-4. The exact terms of the achievement can be found in the game client by clicking on your nickname → Achievements → Summer landing.

Unfortunately you have missed the first few stages of the event to complete them via the mission, however Missed Stars can be purchased in the Achievement window until the 2nd of September, 2021. Don't make a

Yes indeed you can. In game, click on your nickname → Achievements → Summer landing. From there you can see the stars required for the BMD-4 and the taker tasks breakdown.