There are 21 cars, each 4m long, so 21x4m = 84 m gives the total length of the cars.
Then you have to work out the total length of the gaps between the cars, which is
20x 1m, giving 20m.
(The number of gaps between the cars = number of cars -1, so 21-1=20, i.e., 20 gaps. If in doubt, always try these things with small numbers first. For example, if there were 2 cars, there would be 1 gap. If there were 3 cars, there would be 2 gaps, so there is always one less gap than the number of cars.)
The total length of the cars plus the total length of the gaps gives the length of the queue of 21 cars, so
84m + 20m = 104m.
I hope this helps.
I see Chicko-mum has already answered, but it took me a while to write the post so I'll going to post it anyway.