I’ve only just seen your message, but here’s my take on it, despite it being too late for the 4/1!
In your position, I’d take the non-BOG best price, if only one firm is offering it. I’m not a fan of E/W betting really, but I do see where you’re coming from in a way.
If I was to follow your approach, I’d want 4/1 to go E/W at 1/4 odds and 5/1 to go E/W at 1/5 odds, so I got my stake back. The problem with, for example a 7/2 E/W bet is that you sacrifice a large chunk of winnings if the horse wins.
I don’t have any exact figures, but I’d guess that from every 7 selections, I get 2 winners, 3 placers and 2 unplaced on average and I’m not convinced that the three placed returns compensate for the loss of winnings on the two winners, so it wouldn’t be for me.
Personally, I go win only, but on occasions, I will do a 75/25 bet ie 0.5pts win and 0.25pts E/W, so that a placer covers the whole stake on bets at 12/1 and bigger at 1/4 odds and 15/1 and bigger at 1/5 odds.
25/1 and upwards is good for E/W betting, but even then I’d consider 0.5pt win and 0.5pts at Betfair SP.