Boil, if the horse wins, you get £80 return (20x4.0=80......60 quid profit and your 20quid stake back). Remember, 4.0 is the same as 3/1.
If you lay the horse in play at 2.0, that's the same as laying it as Evens. So your liability in this bet is £20, and if the horse does lose, you win the backers stake of £20 plus getting back your £20 in liability.
So if the horse in question WINS- you return 80 quid (4.0 x 20), so 60 quid in profit on that. However, because you laid it at Evens in running, you lose your 20quid liability so your overall profit is £40.
If the horse LOSES, your 20 quid win bet at 4.0 is down the pan. However, you laid it at 2.0 (Evens) so you win 20 quid from the backer plus your 20 quid lay bet liability back. So you get £40 return in total having staked £40 overall, hence the 'no lose' idea.
Easiest way to remember decimal odds is to knock a 1 off the first number:
4.0 = 3/1
3.0 = 2/1
2.0=1/1 (evens)