LTO4 doesn't have media size of 4TiB for sure
LTO4 native is 800GiB, LTO5 is 1.5TiB, LTO6 is 2.5TiB and LTO7 is 6TiB. With compression applied it goes beyond those values, but how much is everyone's guess since it depends on data. So, if your volume contains dumps which are compressed, hardware compression won't be able to do much and you might not move much beyond native values. Second, some drivers and implementation (eg. IBM) require you use their own device handlers - and if not then no compression is applied. In your case, we see compression is applied to other volumes so I assume device handles are ok. So, it is either media content or some external event (like scsi bus reset) which caused full volume marking to appear earlier than expected.