If the product is not in the market, then you can straight forward move on to the new yocto version with 4.x kernel you want. Please refer the i.MX Software and Development Tools | NXP link for the documents of the same.
If the product is in the market and you really want to upgrade it remotely(I guess this is the case you are talking about), then you can try using swupdate feature to upgrade your embedded system. For this you need to update the kernel recipe file and dependency in your current yocto. Then you can update the kernel with SWUpdate: software update for embedded system — Embedded Software Update Documentation 2019.11 docum.... (Note: After doing this you need to verify your product with each and every peripheral and make sure that is is not having any dependency on any package of yocto, as you are upgrading only kernel and not yocto).
