Preheat the oven to 200°F (93°C). Line one or two baking sheets with parchment paper.

Peel the 4 firm bananas. Using a sharp knife or an adjustable mandoline, slice the bananas into 1/16 to 1/8 inch (1.5–3 mm) thickness, keeping slices as uniform as possible.

Pour the juice from the 2 lemons into a shallow bowl. Working in batches, dip each banana slice briefly into the lemon juice or brush both sides with juice to coat; shake off excess.

Arrange the banana slices in a single layer on the prepared baking sheets without overlapping, leaving small gaps for air circulation.

Bake for 1 to 1½ hours. After this time the slices should lift easily from the parchment; if so, use a thin spatula to flip each slice.

Return the flipped slices to the oven and bake another 1 to 1½ hours, checking every 15–20 minutes to prevent burning. Total bake time will be about 2–3 hours depending on slice thickness and oven. Remove when slices are dry and firm but may still be slightly leathery—they will crisp as they cool.

Cool the banana chips completely on the baking sheets. Once cool, store in an airtight container in a cool, dark place. For longer storage, refrigerate or freeze.