I am currently in an IVA and am considering offering a lump sum. I have paid £3K to date. Over 60 months, creditors would receive £9K, would the creditors accept £4K lump as full and final offer.
Not likely, creditors will normally expect to receive 100% dividend in payments, in other words the remaining term in months multiplied by IVA payments. They will also want to know why you are settling and where the funds would be coming from.
If your circumstances have changed and your surplus income has dropped then creditors may accept what is on offer. Otherwise I agree with Winter Blues but ask your own IP.