You can't directly convert "decimals" to liters. Liters are a unit of volume, while "decimals" is a general term referring to numbers with a decimal point.

To convert to liters, you need to know what the "decimals" represent:

* If the decimals represent a volume measured in milliliters (mL):

* There are 1000 milliliters (mL) in 1 liter (L).

* To convert 4200 mL to liters, divide by 1000: 4200 mL / 1000 = 4.2 liters

* If the decimals represent a volume measured in other units:

* You'll need to know the conversion factor between the other unit and liters. For example:

* 1 cubic centimeter (cm³) = 0.001 liters

* 1 cubic meter (m³) = 1000 liters

Please provide more context or the units of measurement for the "decimals" so I can help you with the conversion.