Both are compact loaders on a universal quick-attach plate, so your bucket, forks and grapple work on either. The difference is lift, reach and how much ground the machine needs.
Loader buying comes down to one question more than any other: what is the heaviest thing you need to pick up, and how high does it have to go?
Property owners moving mulch, soil and light pallets around a barn or yard: RS03. Contractors and farms loading trucks, stacking full pallets or running a grapple daily: RS04.
Buyers size loaders by bucket volume and then discover the machine cannot lift a full pallet of block or a round bale. Weigh the heaviest thing you handle regularly and size from there — everything lighter takes care of itself.
If you dump into a dump trailer or a pickup bed, the smaller machine can leave you short. The larger loader clears higher sides comfortably, which turns a two-machine job into a one-machine job.
Both run a universal quick-attach plate, so standard skid steer attachments fit either. Match the attachment's weight to the machine's lift capacity, not just the plate.
If you handle round bales or full pallets, the RS04. For mulch, feed, and general yard work in tight spaces, the RS03 is easier to live with and cheaper to buy.