The extra wand length on the B&G Sprayer 1 Gallon 18 in. Wand (N124-S-18) vs the 9 inch wand would just make applications easier by you not having to bend down as far to apply products. The 18 inch is generally the more popular sprayer for this reason.
It is best to have two seperate sprayers, one for herbicides and one for insecticides. If this is not possible, you will need to triple rinse the entire sprayer, tubing, tank etc after each and every use. Never leave mixed solution in your sprayer, as this can cause damage to your equiptment.
Unfortunately, the B&G Sprayer 1 Gallon 18 in. Wand (N124-S-18) does come with a pressure relief valve. It is located in the actual wand itself.
The tip that comes with the B&G Sprayer 1 Gallon 18 in. Wand (N124-S-18) is the 4 Way Tip 5800 that features 2 sizes of pin-stream and 2 sizes of fan spray patterns. The B G 5800-CC Multeejet Crack and Crevice Tip has a small opening to allow you to insert a small straw to perform a crack and crevice treatment.
The B&G Sprayer 1 Gallon 18 in. Wand (N124-S-18) should be triple rinsed after use and the wand flushed with just water to ensure any residual product is removed and allowed to dry before storing. Be sure to keep out of freezing temperatures so that the brass wand is not damaged.
If you drilled the holes 12 inches apart, then you would have 10 holes over 10 feet that you are trying to fill with 4 gallons. This works out to be a little less than half a gallon per hole. To fill these I would recommend using the one gallon sprayer on a "pin stream" setting so you can force the liquid down the hole and not splash it everywhere. You could use the B&G 1 Gallon Sprayer N124-S-18. You can also use a funnel and pour the termiticide down the holes. For more information on treating for termites, please click the link below.