I can't find 10-6-4 fertilizer or anything specifically for boxwoods. Can you suggest specific commercial products? What about Jobes 4-4-4? I have been using Ironite Plus 12-10-10 every spring.
Certified GKH Gardening Expert
I believe your suggestion of the 4-4-4 will be a good substitute. For a little more nitrogen, you can supplement with coffee grounds as well as a little coffee or tea mixed with mostly water. I do this for many of my plants that just need a little nitrogen. It is much better to underfeed than overfeed. Plus many plants like to be slightly underfed, as doing the opposite would be putting you at risk of salt buildup. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/fertilizing-boxwoods.htm