There's no issue installing DB 4.91 from CFW 4.90.
Depends on how you define losing your jailbreak. DB is 99.9% identical to OFW, so it will not allow you to run homebrew, backups or anything that real OFW doesn't allow, so in that sense, yes, you lose the jailbreak. However, unlike real OFW, you will be able to reinstall CFW without going through bgtoolset again, so in that sense you don't lose the jailbreak - it's just one PUP install away.
DB, being virtually identical to OFW, will not allow you to set the QA flag, therefore you can't downgrade to 4.90 directly, however you can switch to 4.91 CFW and then set the QA flag and downgrade to 4.90 if you wish. Not that there's any reason to.