Wasalam,
We bunch of 4 friends went Quetta and Ziarat from 02 Feb to 05 Feb this month, one of our friend in Quetta told me that Urak Valley and Ziarat got snowfall this weekend and we missed it by one week only 
But overall our trip was great, it was more than amazing, we went in Al-saif Daewoo Coach from old Sabzi Mandi and reached Quetta in around 10 hours, it was comfortable journey as this coach service is outclass.
Coming back to your queries..
Day 1: Leave Karachi 6 am, Reach Quetta till evening. N/S.
It looks good, when going in your own car, its good idea to leave khi as early as possible so that you may hit Quetta before sunset, road condition is pretty good.
Day 2: Leave Quetta for Ziarat, explore Ziarat (please guide me about places to explore). N/S.
In Ziarat,
You may explore its Bazaar, the most obvious Quaid e Azam Residency and if your vehicle is in good condition drive up hill for spectacular view of valley. If you are inclined towards spirituality, you can visit the shrine of Kharwari Baba, which is slightly down the hill. All these places are reachable in car. I think it would be better if you come back to Quetta and stay night there.
Day 3: Leave Ziarat, Visit of Hanna Lake, Visit of Urak Valley (I heard it's not open for public, please confirm and if so from where we can get permission.) N/S in Quetta.
Visit of Hanna Lake is considered as must, we found it in drought condition. As there was no considerable rain/snowfall during this season till the time went. So, we found it bit unattractive. You may visit Urak Valley up to certain point, beyond that point, the valley is under control of Army and to visit that part of valley you'll need permission.
Day 4: Leave Quetta early and Reach Karachi via Sukkur. (Please let me know enroute any attractions and current road condition.)
Sorry, don't know much about this route, as we came back with the same transport.
Do try Lehri ki Sajji and Gulshan Karahi in Quetta. We stayed in Karachi Hotel at Prince Road, which is economical and reasonable hotel. Quetta main bazaar is near from this hotel. For Breakfast, you may try Naimat Kada at prince road. If interested in purchasing dry fruit, Al-Naseeb Dry Fruit is best, with quality Dry Fruits.
One more thing, Do carry enough warm clothes, its freezing cold there.
I hope you'll find above info useful.
