Welcome to BB!
I think they are some reasonably priced ones at Kmart.
$21.99 for 24 grids
You can get them shipped to a local store for free (Orders over $50 get free shipping). I really don’t know anything about the Target grids and if they’re okay or not so I’ll go with Eepster on that. I bought all my grids off craigslist.
Onto your question I think 4 levels would work but as Eepster said it would be tall. For example. Here is my husband next to our 4 grid high cage.
I would not suggest making each grid height one level as you want your rabbits to be able to periscope (stand on hind legs). So for 4 levels I think you would need to go even taller than 4 grids… Which would require a very tall person to be cleaning and getting into the cage. I think you’re better off building two separate cages until they are older, spayed/neutered, and bonded. At that point you can make them a joint cage and you don’t have to worry about it being able to be separated. Anything you build now would likely have to be re-done once they are older and bonded (If that’s your goal). Rabbits can’t usually live in the same habitat after being bonded as they did before.