Because the money in the bank will pay 4% interest on the whole 3 year term of the loan (assuming it is 3yrs here).
With a loan the amount you pay interest on will decrease. So at the end of teh loan you will be paying less £ in interest because the value you are paying interest on is lower.
So if you have £5000 in hte bank, and assume that you get 4% pa interest and that it is calulated annually (for simplicity - as they are usually calculated monthly/daily now) you will get
4% interest on £5000 in year 1
4% interest on £5200 in year 2
4% interest on £5408 in year 3
So at the end of year 3 you will have £5624 in teh bank. Profit of £624
If you borrow £5000 - and again assume that you pay 4.9% pa interest and that it is calculated annually at the beginning of the year (to simplify things), and that the monthly repayments are 1/36 of outstanding balance for yr1 1/24th for year 2 and 1/12th for year 3 (these assumptions are to make the calculations easier...and illustrate the priciple...in RL interest is calculated monthly and monthly payments are fixed at the begining of the loan but my head can't work that bit out!)
Year 1 £5000 outstanding £245 interest charge. Pay £146 per month.
Year 2 £3493 outstanding £171 interest charged. pay £153 per month
Year 3 £1828 outstanding. £73 interest charged. Pay £158 per month. Loan is paid of.
Total interest charged is £489
So buy leaving your money in teh bank you have made enough profit to cover the interest charges, with £135 to pay teh car tax (as long as you don't buy an SUV!)
Basically a small difference in the interest rate on a loan and savings can be soaked up as the amount in £ terms of interest on savings increases with time. The amount of interest on a loan in £ decreases with time.
However....I think that you would have to be lucky to get a loan at 4.9% and savings at 4% TBH....