1. Deflation isn't fixed. Lenders have to make a guess about the spectrum of possible future rates of deflation, inflation, or even the abandonment of the currency.
2. Lenders seek profit, not revenue. 6% interest on 4% deflation is hardly greedy. You're ignoring the risk/reward ratio. Deflation causes defaults because existing loan costs become more onerous. 6% on 4% would almost certainly be a net loss after defaults. The high risk of default during deflation is part of why interest rates hit 18% in the 1980s.
3. Deflation is typically a product of most people having less access to your currency. Debtors would make an estimation that they could afford your loan at a lower-priced Bitcoin (in currency and labor costs). They'll be more likely to default when the currency is more scarce either due to the worse exchange rate of money or exchange rate of labor.