The calculation is done as follows:
1. Add management fees which varies from bank to bank, but it is usually around 2000
2. Total base amount = 45000 + 2000.
3. Total interest = (45000 + 2000) x 4% interest x 5 years. Which means the total interest due will be 9400.
4. Total amount due = 45000 + 2000 + 9400 = 56400
5. Monthly instalment = 56400/60 = 940

SO, you are fine according to the showroom's calculations