If you're hauling with a pickup and have a trailer capable of carrying 4 atvs then you probably do need the sticker, weight wise and MGVWR wise.
Check the manufacturers stickers on both the truck and trailer. If the MGVWR exceeds 4500 kgs when adding the truck and trailer together, then you need stickers on "both" truck and trailer. If you say your trailer weighs roughly 5000 lbs, I would think your truck weighs at least that much, which would also mean you require stickers on BOTH truck and trailer.
The only other time a truck and trailer require stickers is if the RGW of the truck is over 4500 kgs, which is indicated on the registration.