Long drives with a toddler are mostly fine right up until they are not. The difference between a good journey and a miserable one is rarely the distance. It is whether you packed for the twenty minutes where everything goes wrong at once.
Start with the things you will need within arm's reach. A change of clothes, wipes, a hygiene kit, and water. These go in the front, not the boot. If something has to be reached while moving, it cannot be in a bag under three suitcases.
Plan for the stop you cannot make. Every parent has been on a stretch of highway with no exit and a child who needs a toilet now. This is the single most predictable emergency of any road trip and the easiest one to prepare for.
Food is a timing problem more than a quantity problem. Small things, often, spread across the drive. A large snack at the start means a hungry child two hours later with nothing left to give them.
And build in more time than the map says. The drive is not four hours. It is four hours plus however long your child needs, and being relaxed about that is most of the battle.