What is the significance of the 2 and 4 positions in 2,4-DNP?
Probably any DNP could work if you could get it.  But, because of the directing  character of substitutents in electrophilic substitution you inherently favor the 2,4 isomer:


the hydrazine group (or more accurately the chlorine atom which is converted to a hydrazine group) is ortho/para directing.
the nitro groups are deactivating and meta directing.


So you get the nitro groups meta to each other and ortho/para to the hydrazine group.  Therefore 2,4 DNP.