When it says 'recommended maximum 4.xx BTC', am I save sending up to this amount (will I get the listed exchange rate if I dont go above this?)
If I send it 4 BTC, is this a lot riskier (in terms of maybe getting a worse rate) than sending just 1 BTC?
BlockTrades keeps a pool of each asset it sells for fast delivery. The suggested max deposit is a fraction of the amount in the pool (1/5 at the moment, if I recall correctly). The only reason we report it is just that so we don't get too many large simultaneous orders that exhaust the pool too rapidly. So it has no real relationship to the exchange rates we quote (see below for how we compute exchange rates).
We've set the suggested deposit limit quite conservatively at the moment, in part just so that no one would place really large orders early in the site's life. As we continue to optimize our internal buying algorithms, we'll likely relax the suggested deposit limits.
Our exchange rates are set by how much it costs us to "replace" the order. When you fill in an amount you'd like to buy, we look to see how much it will cost to buy up that amount from the market. So you'll notice that if you increase the amount you want to buy to a large value, your exchange rate will go down as we start to have to buy up more "fringe" orders. Eventually you may even get a message "insufficient market liquidity", which means we can't buy enough to satisfy an order that large.