The official currency in Bali is Indonesia Rupiah. The currency notes available range from IDR 1000 - IDR 100,000, whereas, coins available range between IDR 100 - IDR 1,000.
If you are going to be in Bali for 4-5 Days, exchanging money at one of the ATM's is a great idea. You can either use a credit card, travelers cheques or the local currency to transact in Bali. Keep in mind, however, that your local bank may charge convenience fees to exchange the currency.