circuit of 2-4 decoder
It is possible to build a full adder using 2:4 Decoder with an extra Enable input.
You will need 2 2:4 decoders, a NOT gate and 2 4-input OR gates :
This can be done by giving inputs a,b to both the decoders and '~c' as an enable input to the 1st decoder, and 'c' as an enable input to the 2nd decoder. The outputs lines 1,2,4,7 are OR-ed to give the sum, and the output lines 3,5,6,7 are OR-ed to give the carry. (The output lines are numbered from 0 to 3 for the 1st dec and 4 to 7 for the 2nd dec).
If you are not allowed to use a NOT gate, then you would require 3 2:4 decoders, where an extra decoder is used to implement the NOT function.
Hope this helps.