I'm going to assume a few things since they weren't provided. We'll assume 4% interest(as per your question), 30 yr amortization, and 3.5% down(FHA Loan) as that is probably the least amount you can do. This will give you a conservative number based on you putting down as little as possible. You will have to pay MI(mortgage insurance) and UFMIP(upfront mortgage insurance at 1.75). Your best bet is to use the seller's assist as 3.5 % down plus 2.5% towards closing costs to minimize your out of pocket expense at closing. So Loan amount is $135,100 plus UFMIP of $2364.25, so total loan amt is $137,464.25. Now MI is going to be 1.25. So 135,100 x 1.25 = 1688.75 divided by 12(months) = $140.73/mo. Based on terms mentioned the monthly principal and interest payment is 656.28 + MI of $140.73 means a total lending payment of $797.01. Don't forget to factor in real estate taxes and home insurance also.
Hope that helps.