Nucleophilic aromatic substitution on pyrimidines: C2 vs C4 selectivity
An explanation can be provided by frontier molecular orbital theory.  The LUMO coefficient is higher on the C4 vs C2.  This is a general effect meaning that an EWG para to the leaving group is more reactive than ortho.  See 4-halopyridines vs 2-halo and 4-nitrobenznes vs 2-nitro.  There could also be an effect from the repulsion between the nitrogen lone pairs and the approaching electron rich nucleophile, which would be greater in proximity to 2 nitrogens rather than 1.