Hello oolongX!
NO FEES means that you need to have some additional amount ( 1 coin is quite enough or even less) for tx fee. So if collateral is 4000 so you'd better have 4001 HLD or 4000.1hld
 
2nd  masternode collateral tx is to be done this way:
Go
Settings->Options->Display
Check up "Display coin control features", then Apply, then Ok

Go to 'Send' Tab
Click 'Inputs...'
Check 'List mode'
Using Right Click lock your existing MN collateral coins that as they couldn't be spent for other tx.
Now you can make any txs.