First of all your roof is not a 4" thick. It will be 8-10".
when a roof is designed the dead and live load is taken in account. the dead load as well as live loads are factored for safety or unknowns.
excellent material was used buy how it was used? I mean was the concrete good? proper W/C ? what is the strength of concrete. for our domestic concrete it should be min 3000PSI. Also was the rebar was placed according to design or the "lanter wala" wala just placed the steel according to his wish/will or his own experience. BTW in actual the word is "lintel" and we made the world "lanter".
anyways well over short spans I don't see major issues. On longer spans these must be designed accordingly for extra weight. but the thing is weight of soil is not that much until it gets moisture. Most of roofs in Pakistan are designed to get the "chokay wali eent" and screed. So For existing roofs with this "Chooka" already.. I will not recommend green roofing. This is increasing the weight a lot.
The weight of green roof somewhere will increase for 2-4" soil in the range of 20-50 lbs/sft..
If your structural slab is still there, I mean its bare concrete structural slab with no "choka" I don't see any problem doing light green roofing. Like 3-5" top soil with insulation/ filter fabric,drainage mat. Off course a proper waterproofing before all this goes on top of it.
I will not recommend extensive green roofing like vegetation with 10-12" thick soil. This is a lot of weight and when its gets wet it will increase by 2 to 3 times. If you want You can do it easily if the engineer have already designed your roof and supporting members for it. I mean new construction with this thing in mind.
Verdict: light green roofs with 3-5" top soil possible on existing roofs.
