That why he have higher BR, than 39/25T, with Vikhrs and Mercury target pod (no thermals, and still only frontal view/targeting, but with better angles than stock TNS).
In this case - MAW more trustworthy than IRCM, that just ignored by most top-tier fox-2 with IRCCM.
4 missiles is better than 2, but if this “extra 2” is R-60M - it’s not a big deal. 16 Vikhr ATGM, with their proxy fuse been even more dangerous in head-ons.
But if Su-39 will got it’s R-27ER/77, guided by Kopyo (Spear) radar pod - setting it on higher tier will be pretty legit.