As said some time ago about army cap and population cap =P, tho you could have more than 1 army. So if 1 army gets killed you still can use another one, as they said getting killed one army would not be major pushback.
And about 4 armies attacking you ... as said, that person should have 4 accounts (he can have 4 castles in 1 account but must switch them to control). Beside that there is also limit for numbers and lvl for attacking ppl, so it cant happen 100vs1 or high lvl vs just created castle. I am not sure if ratio for participation is just 1v1 or there could be also 2v1 or little more. Ofc as i imagine if 1 player is higher lvl than he could be attacked by more players lower lvl.
Picture made by Aametherar the orc
Glory to the elves!