Sales entry with single VAT rate.

Press F12 -> select Invoice/orders entry configuration -> press enter -> set yes at the option 'Use common ledger for item allocation?' -> press enter.
At Sales @ 4% A/c, select the sales ledger with VAT rate applicable like sales @4%.
Go to Accounting Vouchers from Gateway of Tally and press F8 for sales. There select Sales @ 4% under sales ledger and select sales @ 4% under VAT/TAX class. Enter the item details there. Then after that press enter and select VAT ledger, for example output VAT @ 12%, from the list and provide the rate to be charged. Press Ctrl + A to save the enty.