Reddit mentions of JW Pet Comfy Perch For Birds Flexible Multi-color Rope

Good start and I commend your preparedness!

The hamster litter try is probably too small for rats because they'll want to fully step inside to do their business. A ferret litter tray is a better size or you can just use a pyrex pan or whatever you have lying around or can nab from a goodwill.

The carrier looks good. I'm a fan of top loading carriers. They're so much easier for getting scared rats in and out.

Cargo net looks great. You can find cheaper tunnels/tubes at the hardware store ($4 flex drain). They're not clear and they don't come in cute colors, though. The hut is fine but you can also just use cardboard boxes or overturned organizer bins or baskets from the dollar store. A lot of cage accessories can just be sourced from the recycle bin, your basement, or dollar stores and thrift shops. A few of my favorite store bought cage items include Space Pods, lava ledges, and bendy rope perches. I keep multiples of each in my boys' cage.

Toys and chews seem fun but a lot of rats just ignore them. I wouldn't spend too much money right away on these types of things.

Hammocks are great but they can get destroyed so I make my own. It can be as simple as just clipping up random scraps of fabric. They don't have to be fancy or pretty. I keep several hammocks in the cage so everyone can be alone if they want to. Lots of hammocks also makes good use of the space so your rats can take full advantage of the giant cage you're getting them. I also keep multiple sets of hammocks because they have to be swapped out and washed very frequently. I wash all of the hammocks and liners at least twice a week.

I'd recommend getting two water bottles. Sometimes they leak or malfunction so it's great to have a second (or third) one attached to the cage as insurance.

You don't need to use the Oxbow Young Rat. Babies can have the adult formula and you can just give them some extra protein-rich foods like egg or chicken a couple times a week. Also, Chewy has better pricing on Oxbow. With 4 rats, you can get away with purchasing the big 20 lb bags to save a little extra money. It'll still last around 5 months so it's good to store it in the freezer.

If you're planning on using fleece liners, you'll need an absorbent layer under the fleece to soak up the pee and help control odors a little bit. UHaul padding is a popular choice but you can also use incontinent pads, towel, newspaper, etc. Anything than can be washed or thrown away very frequently. Fleece liners are reusable but they're the absolute worst for controlling odors and ammonia so they need to be washed very, very frequently. I use fleece/UHaul "mats" on the levels of my boys' cage and aspen shavings on the floor. The fleece needs to be changed/washed 5 to 10 times more often than the aspen. Also, some rats can be voracious chewers and will absolutely destroy fleece so you may not get as much use out of them depending on your rats. If you want to use a loose substrate bedding on the floor of your cage, you can pick up a large concrete mixing tub from Home Depot for $13 and it can be fitted into the frame of the cage (like this). It's very deep and can keep a good layer of bedding. Bass Equipment also sells metal pans that fit the Critter Nation. They're not cheap but they're wonderful.

I believe it was this: JW Comfy Perch for Birds https://www.amazon.com/dp/B0002AR71S/ref=cm_sw_r_cp_apa_GnzMAb56TC611