What to Plant in October in Zone 4

This month is a normal window for cloves, not a last resort

Garlic and shallot are the only 2 crops still open this month, and the deadline that actually governs them is the ground freezing solid, not the first light frost that already passed weeks earlier. Both need 4 to 6 weeks of root growth in cool, unfrozen soil, and here that puts the real target squarely in the second or third week of this month, a completely ordinary window rather than a late scramble.

Plant slightly deeper than you would in a colder zone, about 3 inches to the tip of the clove instead of 2, which buys a little extra insulation for cloves going in on the later side of that window. Firm the soil over each clove with your palm rather than leaving it loose, since loose soil traps air pockets that dry out roots before they establish, and a clove that fails to root well before hard cold arrives is far more likely to heave out of the ground during a freeze-thaw cycle.

If the calendar slides past the third week, keep planting rather than waiting a full year. A clove that manages even 3 to 4 weeks of rooting before the ground locks up still has good odds under a thick mulch layer, even if the bulb next summer runs a size smaller than one planted right on schedule.

Mulch heavier here than the calendar suggests you need to

A garlic bed planted this late benefits from 4 inches of straw mulch rather than the 3 inches that works fine on an earlier planting, simply because the roots underneath have had less time to anchor. Add that mulch within the same week you plant, not weeks later once the ground has already started to cool hard.

Check the bed again after the first freeze of the season, whenever that lands. A gap of bare soil wider than your hand is worth patching immediately, since a shallow-rooted late planting is the one most likely to heave in an early winter freeze-thaw swing.