It's going into quite different territory than the first three chapters, and becoming darker. The social comedy and keenly-observed dialogue is still there, particularly in the character of the homosexual dealer Caldicott (it would be an anachronism to call him 'gay', I think) but it's no longer the central focus. Hurtle's wartime experience is dealt with briefly, and is largely a framing device for the sequence of letters he receives at the start of the chapter. (The letter from May Noble, the Courtneys' cook, is a comic gem.)
Instead, we get into Hurtle's attempts to wrestle with his art. It's all very Heroic Modernist Bloke: Hurtle uses a couple of advances from his dealer to buy land near Hornsby, builds a shack and wrestles with his muse.
Personally I'm a little skeptical about this way of looking at art, but that's not spoiling my enjoyment of the novel at all; and White obviously takes the myth with more than a few grains of salt, explicitly contrasting Hurtle the stinking bushman with his rather Fauntleroyish boyhood, and acknowledging that the latter is just as important to his art.
Hurtle's also literally a ponce, living off Nance's earnings; this is a term which by the time I heard it as a kid in the 70s had come to simply denote effeminacy and had lost any direct connection to a man who lives off the earnings of a prostitute, but I'm sure it wasn't considered exactly manly in the 20s.
This way of looking at artistic creation is also a bit dated, I feel; I suppose that Brett Whiteley was the last of the Heroic Blokey Artists, although it lives on in twilit form in a figure like Michael Parr. The next celebrity Australian artist-bloke was Ken Done, and his heroism was financial and corporate, not psychological or even aesthetic.
The sections at Ironstone are a little like the opening chapters of The Tree of Man, in a more nightmarish light. It's a long time since I read the latter but there's a memorable image in it of a man drinking tea by his campfire, and I think I detect an echo of this image in the closing section of Chapter 4, with rotgut brandy taking the place of tea.
This sequence also inverts Hurtle's precocity in the opening sections; now, as an adult, he regresses to the most infantile means of expression imaginable, smearing shit on his own self-portrait.
I'm not entirely sure what I think of the depiction of Nance and Hurtle's actual sex - there seems to be a lot of disgust and the writing style gets a bit purple and evasive - but their broader relationship seems to be to be well-drawn.
Chapter 5, by contrast, is just a vignette to establish the passing of time after the preceding chapter and let us know that Hurtle is starting to build a reputation overseas. It's really the first time we see him from the outside, as he has a halting conversation with a grocer in a public park. The grocer then goes off and wanks into the shrubbery. I wonder if that sort of thing was more common in the 20s than it is now, or less? People have much more privacy in their home life than they used to, and I'd think there would be fewer men sneaking off to the park to eavesdrop on the couples to get their jollies. The grocer today would be looking at porn online.