Just because a backpack-sprayer has 4-gallon capacity doesn't mean that you have to fill it!
Place it empty on an inverted barrel or other flat area at your waist height, oil the crank parts, and put only one or two gallons in it.
Use only products that stay suspended easily.
If you use Sulfur, or any other product that sinks, or if you use oil, which floats, you will have to do a lot of shaking and dancing, which are very tiring.
This tends to be true even if the sprayer comes comes with an "agitator" attached to the pumping column in the tank, that only works at medium depth, and only when you pump with strong, long strokes.
Solo sells a four-foot wand that telescopes to 8 feet. To mate it to other brands requires adapter parts not sold by Solo.