You should b OK with your mix. Just focus on packing your corners properly so the wicking action takes place effeciently. You want damp soil; not wet for the corners and you don't want too many air pockets so layer the damp medium and pat it down gently. They recommend to use a potting mix it because they want u to buy their nutrient blend.
In all honesty there is no such thing as potting soil, potting mix etc. Just dirt with different ratios of amendments innit. Dirt with good stuff that doesn't turn into mud = soil. I suppose. That's a rabbit hole.
Sunshine mix #4 is peat based with little trace element innit good for seeding and starting ; Good for micro-greens if ur into that. After maybe 4th watering it will once again be an inert peat media. I amended it with a plethora of things. Since it is peat and as it breaks down acidifying the rootzone I followed their rule of dolomite layer, medium, mix, nutrients, soil mound. I did not like the trenches of nutrients so I actually mixed it all in half way. U will find what u like.
sorry 4 thread jacking it was an accident. I am subb'd now