In order to upgrade to the Tier 4, you need any two of the T3s from the same tree. If you scroll down in the recipe window for each T4, there are other recipes. All combinations are listed, just find the recipe appropriate to the two T3s you made.
It can be any two different rares from the same tree. Muscular Total Core Revamp can be paired with any other rare Muscular to make either or the two very rares.
The scroll bar on the very rares goes quite a bit further, revealing 6 different recipes. Only limitation is that you can't use two of the exact same rare (2x Muscular Total Core Revamp), or rares from different trees (musculature rare + cardiac rare).
"My inner mind has become a reality-cracking overgod. He torments me! Help!"
Looking at the items needed to make your top level alpha slot it looks like making certain tier 3 ones are required. For example it says that if I want to make the Muscular Total Core Paragon I need both the Partial Core Revamp and the Partial Radial Revamp. Does this mean tht if i made the Muscular Total Core Revamp its basically just a wasted alpha slot? I know you can swap them in and out but it seems a waste to even allow players to reach a dead end in terms of upgrading alpha slots and have to start over.
Dragonhawk (lvl 50 fire/energy blaster)
Flamefrost (lvl 50 fire/ice blaster)
'Snowflake (lvl 50 ice/dev blaster)
Dragons' Flame (lvl 50 fire/fire tank)
Psyshock (lvl 30 Rad/Psi defender)