Here is a set of worksheets, suitable for year 4 children, which looks at multiplication by 4 using mental methods. With 2 and 3-digit numbers often the easiest way to do this is to double the number and then double again. This is just one of the many strategies that children should use when working 'in their heads'. It works really well for some numbers; eg 210 x 4:  Double 210 to get 420. Double 420 to get 840 but is not necessarily always the best way. eg 30 x 4 can be worked out through knowledge of the 4 times table and that 3 x 4 = 12, therefore 30 x 4 = 120. The more strategies children have the better, so keep practising!