Locked memory is used to stop memory being paged out to the swap file.
Rclone doesn't use it, so I'm afraid this is probably not the problem.
In general processes only lock small amounts of memory (like the keys for crypto) so I don't think you should change the default.
Your ulimits looks fine so I think we have to look elsewhere for why rclone gets killed when it approaches 4G of memory use.
You are running 64 bit kernel with 64 bit rclone so there should be no reason it stops at 4GB.
Could systemd be limiting it? I don't see a memory limit in your systemd config file but maybe there is a place to put it I don't know about.
Any ideas @Animosity022 ?