To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current Auction Price and Previous Prices: The current auction price is 4000 euros. In previous auctions, we have seen hammer prices that vary, but there are examples of coins with similar conditions that have sold for prices above 4000 euros. For instance, at the auction on 2020-03-12, a coin with a condition of 95 sold for 4000 euros, and in other auctions, coins with conditions of 85 and 90 have reached hammer prices of up to 3600 euros.

3. Price Trends: The average hammer price for similar coins is approximately 2934.31 euros, but there are examples of coins that have reached significantly higher hammer prices, especially in higher conditions.

4. Demand and Supply: The coin has been auctioned 42 times, with a sell-through rate of 80.95% (34 out of 42). This suggests that there is consistent interest in this coin, which may influence the price.

5. Price Projections: Given that the condition is good and considering previous hammer prices, an estimated maximum purchase price could be in the range of 5000 to 6000 euros, especially if there is interest in the auction.

Final Estimate: Based on all these factors, an estimated maximum purchase price for this coin could be 5500 euros. This reflects a balance between the condition, previous hammer prices, and market demand.