I'm pretty sure EAS reactions on pyridine are typically done under pretty rough conditions (couple hundred + oC under pressure, conc. acid, etc.) to overcome the inherent difficulties (still with pretty poor yield). I'd expect the conc. acid to tie up the basic N-LP, and I'd expect substitution to take place on 3/5 due to higher e- density than 2/6 and better resonance structures than on 4.