Happy to hear you are enjoying the game.
1. The 4 Stamina is your starting Stamina for the custom character. When choosing Starting Abilities, look at the Stamina cost to use each. Then, you pick up to your total starting Stamina worth of abilities for your custom character. So, a "tier 4" character would have 4 Stamina point to "use" for picking Starting Abilities from the existing characters. In most cases, you will get 4 Abilities as most only cost 1 Stamina from the Starting Abilities cost 1 Stamina to use.
2. An Unlockable refers to Abilities that you only unlock with the main characters during gameplay. You can see them on the bottom of each character under "Unlockable Abilities." With a "tier 4" character you pick abilities from these lists up to the 4 Stamina cost total. Choose what you want, calculate the total Stamina to use them, and then adjust (same as above). You unlock these the same way as the base characters, i.e. you need to do Training and spend coins.
3. When you are rewarded "Equipment" pick either table to roll on. It is split this way to make it easier to get things you want as some Delves require items from specific lists.
Let me know if you have any additional questions.