I presently use both. The 4 bar is OK for grouting and sinks and just about suffices for most general cleaning I smaller kitchens so you could get away with it. Tiles and canopies are
steam mopped at lower pressure anyway so it doesn't matter so much. Where the 8 bar really comes into it's own is on ovens and friers. You really do notice the difference. Fit a metal scouring pad over the steel detail brush attachment and the carbon, browning and grease just dissolves before your eyes. It also works more rapidly so cuts down on time. Chemical injection is useful on floors and walls and some heavier grease accumulation but to be honest I seldom use that function (8 bar does a great job on chewing gum on hard surfaces).
Be aware that more steam = more water so the oven may drip for some time afterwards. The so8 has a 2.5 litre tank and the so8 a 5 litre one, so you'll be topping up more often with the so4, although they both have the benefit of continuous fill.. Neither machine has chemical injection. You need the sdv4 or sdv8 for that.
Don't imagine that the steamer will get rid of everything. That really gummy yellow goo that covers the edges of the friers and ovens still has to be scraped away first (trust me I've tried every chemical known to man on it).