I have -4.00 for both eyes, single vision only,so what you suggest me to get best for myself,( it is just a normal prescription with -4.00 near sighted with no extra astigmatism prescription).

it's a bit strong,but you don't have to choose the 1.67 super thin lenses.1.56 index or CR39 lenses are alight for you.

I have a strong prescription which is SPH-7.00/CYL-1.25 for both eyes, what kind of prescription lenses will suit me? If 1.56 lenses are suitable for me or I need the 1.67 lenses? Your reply will be appreciated.

I have the similar prescription with you, I think 1.67 lenses are better for you.