I know i would have an outbreak of diatoms pretty bad.
My guess is that 300lbs-350lbs of yardright sand would come close to a 4" depth. Thats only 6-7 bags or $90-$105. ALbeit that same sand up here is like $5 a bag. Not sure why its 3X more down there.
Silica sand will not cause you Silicate problems. They are different. If Silica was a problem then every glass tank would have diatoms all over the glass surface of the tanks.
The only real negitive to silica sand that I can point out would be that often silica sands grain size is larger than the yardright or southdown sand size and the particles are often sharp on the edges compaird to round.