Acceptable losses depends on how anal you are. The fight you mentioned, was it one you started or did he chase you down and started it? Did you capture the lord, got great loot and the xp from it did it level your troops and how many got levelled? Even then one knight out of fifteen to twenty is nothing really, their purpose is to serve you and your needs or die trying.
And then since you're fighting for a faction, beating the previously mentioned Lord weakened the enemy so losing four high levelled troops is nothing, as long as they (the enemy) got wiped out. Not entirely true, if it resulted in you loosing majority of your troops I'd see it as a meaningless battle but I wouldn't reload...live and learn.
Did you fought the Lord because he was besieging a friendly castle or did you fought him to take a castle and resulting in you gaining this for Veagir (sp?). If so losing four soldiers is perfectly acceptable.
Concerning sieges no AI can beat the ferocity of the player, if you don't use ranged weapons (like me) make sure you get on those walls and start slaughtering to make room for your troops...they seem to think its a good idea to stand in a line and get butchered by the defending forces instead of actually pushing through. When you and your footsoldiers is up there get your archers on there to assist (with arrows/bolts) when they (footsoldiers) go down those "thin" stairs trying to get to the courtyard (depends on the Castles layout). Archers out side of the walls waste their arrows/bolts on the wall and only kill the occasional troop...at least in my experience so I might just be cursed.
No one (I think) likes to loose soldiers but its just something one has to accept, fighting in the open can "spawn" unfavourable battlefields, attacking a castle without knowing how it looks for whatever reason can have bottlenecks that will kill a bunch of your (suicidal) troops.
Then again last time I played native was the beta back in 2005, recently re-installed Mount & Blade when I found my serial by accident and I started using mods right away. So take my advices with caution.