Is a concrete slab required to support a block wall?
1) Part of the wall will be built on what is currently dirt in a planter area; the rest of the wall will extend onto existing pavers. Do I need to pour concrete in the planter as a foundation, level with the pavers? Or can I get away with compacting crushed gravel as a base in the planter?


You'll need to do 'something' to deal with the overturning and sliding forces on the wall, not just from the soil pressure but also that significant 1,600# surcharge. A 4 feet tall is generally the start of the retaining wall design height and even below that height it's assumed that the wall has a standard footing. You will need to provide a concrete footing and tie it into your wall with rebar (steel reinforcing). A typical footing (for 4' of soil) is 16" wide and 8" deep w/ (2) #4 rebar continuous. You need to have the bottom of the footing below frost depth to avoid issues with frost heave, and for soil in front of the footing to prevent it from sliding along with the entire wall. Provide a footing drain at the back or weep holes at the base of the wall because any water building up creates a lot of additional force that isn't considered


  2) The grade of the pavers angle slightly down, toward a drain. What's the best way to level out the base of the wall that will extend onto those pavers?


The footing should be below the pavers so you can have the pavers butt right up to the finished wall