I'm planning to grow spicebush (Lindera benzoin) from seeds
in "plantable" pots now, and then put them in the ground in late
summer, when our house will be built and it comes time to plant
the rain gardens. As a gardening novice, I have no idea how
fast the seedlings will grow and whether a four-inch-diameter
pot will be large enough to contain one of these plants for so
many months.

A few minutes of Web searching brings up diametrically opposite
statements about the growth rate of this species: "grows quickly"
vs. "slow-growing shrub." (It's exasperating how horticultural
literature aimed at a gardening audience can be so anecdotal!)
Now, I know from my wife's vegetable gardening that a tomato, say, would outgrow a pot that size pretty soon. What about Lindera?