Hi Chilli,
I am confused as to what you want.
As long as the price has dropped since your initial back you can create a free bet at any odds by laying £4 (ie getting your stake back if the selection loses). The lower the price when laying the better the profit. So in your example of a £4 back at 5.4 the following it true:
Odds Stake Profit Loss if loses
(Lay) (after comm)
5.0 £4 £1.52 £0
4.8 £4 £2.28 £0
4.5 £4 £3.42 £0
4.0 £4 £5.32 £0
3.5 £4 £7.22 £0
etc
Given that fact, please can you redefine what suggests you want to lay at 4.18 ?
If I am being stupid perhaps someone else can help?