Arrange in decreasing order of basicity: (I) 4-aminopyridine, (II) 3-aminopyridine, (III) pyridine, (IV) 4-nitropyridine.

The ring nitrogen is protonated in each case. The $-\mathrm{NH}{}_2$ group at C-4 can push its lone pair to the ring nitrogen (+M), strongly raising basicity. At C-3 it cannot conjugate with the ring nitrogen, so only a weak effect operates. Pyridine is unsubstituted, while the nitro group at C-4 withdraws electrons by -M and -I, making it the weakest base. The order is I > II > III > IV.