It is not lawful method. It is interest. However there is one method to make it lawful and save oneself from interest that one lakh rupees which one has to pay as interest in paying the price of 4 lakh in instalments, so you should fix the total amount of five lakh rupees as total price of the car and will say that you were buying this car in five lakh rupees and you will pay the price in instalments. Thus you can avoid interest and purchasing of the car shall be right as per the Shariah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband