If you gave an 8yo 5mg singulair (montelukast) accidentally, vs. 4mg, is it no big deal? (the 5s are the younger sib's- sitter error). why do they come so close?

No problem: This should not create a problem. You are right, it is confusing having doses so close in strength.

No big deal: Singulair (montelukast) is a very safe drug. I have been using it many years in practice. The dosing I have always found curious myself, dosing the 4 vs 5 mg. I welcome other Healthtap docs who know the specifics, but in general dosing is based upon the studies that the drug company utilizes to submit for FDA approval. I would just switch them both to 5 mg, for simplicity.