Hello assortedlives,
Like Yan explained the brushes are similar but from a different grade. As a beginner you might want to not spend too much money on a brush but still have a decent brush – that’s the Initial #4.
When you are on a level where you want to invest in a good brush, the 0903WsWl brush is a good one.
Compare it with like buying a t-shirt. You need a new tshirt but you decide if you want a good one for a okay price or you want a good (to better one) for a more expensive price. It’s still just a tshirt only the materials and work put into it can differ.
Hope this helped you, if not please let me know 🙂