I've run out of alchemy storage room on the start of Chapter 4 Lakeside. First time this happened. Do I just store other alchemy items that are duplicates at an Inn, or is there another option? Thanks

You can store at the inn, make potions or sell them...

Go to the village blacksmith. Talk to him. Click the hammer icon. Besides crafting meteorite ore swords, this functions as a selling to vendor service. You can sell a wide variety of alchemical components to this smithy, including fisstech. Abigail also buys some alchemy parts. Brew a variety of potions to keep Geralt going through your game cozily. And discard those that you cannot sell, or do not have secondary substances (albedo, nigredo, rebedo). Your potions are very nice when they have secondary properties.

Being picky, equals to saving storage.