His first example 1♠ - 2♣; 2♥ - 3♦ is I guess open to various meanings nowadays in 2/1 GF, where the situation is already forcing (for us it would request a stop).
But his second and oft quoted example 1♦ - 1♥; 2♣ - 2♠ is the essence of things.
Norman Squire said:
South holds: ♠J64 ♥AJ854 ♦K8 ♣QJ7
North opens 1♦ and, over 1♥, rebids 2♣. What can poor South say? He is being asked to make a limited bid, and can’t. His hand is worth 2NT, but his Spade stop is missing. Either he must guess blindly or he must bid the fourth suit. I contend that the only bridge bid is 2♠. It is a low reverse here but should certainly not be passed. South thus creates his forcing situation without having limited his hand. North must now make the limited bid. He answers naturally, but he must know the possible implications of the bid of 2♠. One thing is sure: South has not a balanced holding with good Spades and Hearts, such as : ♠AQ73 ♥AQ92 ♦82 ♣Q62.
With that he bids 3 NT at once. So the bid of the fourth suit automatically denies the ability to make such a limited bid. South has either a Spade suit or is worried about the Spade suit. ‘Which, North cannot yet tell. But long Hearts South certainly has. Try to construct a hand with only four Hearts which makes this bid of 2, which cannot better give jump preference in Diamonds, raise Clubs, or bid No-Trumps quantitively. So North will give preference to Hearts if he can. Otherwise he will rebid one of his own suits or NT. Therefore, it is clear that he can hardly bid No-Trumps unless he himself stops the Spades. The worst he can have will be three small Spades, when No-Trumps will assuredly be the best spot.
There is nothing artificial or conventional about this bid of the fourth suit. It has a natural meaning and an equally natural alternative meaning. South may have a freak and be plugging on to slam in Hearts or , Spades regardless of North’s next bid. All he has done at the moment is to pass the buck, making an unlimited bid and creating a forcing situation.
I almost wrote above 'the essence of the convention', but Squire insists that "there is nothing artificial or conventional about this bid of the fourth suit" (so presumably it should not be alerted under WBF policy) as it 'has a natural meaning and an equally natural alternative meaning'.
What do you think about this argument that 4SF as described here is natural? If you agree, would you still agree when 4SF creates an unconditional game force? Is 4SF alertable in your RA and if so, is that because it is not natural?