Lewis structures of S2N2 and [S4]2+
Draw each ring with one double bond and the two atoms having no double bond with two electron pairs instead of one.  Verify that the total electron count agrees with two positive charges on $\ce{S4}$ and zero on $\ce{S2N2}$.  The double bond and the extra electron pairs on the two atoms are the six ring electrons in the Huckel rule.