What amount of permafrost carbon release could be expected with the IPCC worse-prediction of a 4.8C temperature rise?
Schneider von Deimling et al. in Estimating the near-surface permafrost-carbon feedback on global warming Biogeosciences, 9, 649–665, 2012 evaluated carbon dioxide and methane release under four senarios, the worst being RPC8.5.  

Under RPC8.5, they calculated release of carbon from permafrost through 2100 of 63 gigatons (Gt) as CO2 and 0.5Gt as methane.

Through 2200 (cumulatively): 302Gt as CO2 and 5Gt as methane.

Through 2300: 380Gt as CO2 and 12Gt as methane.  

Other estimates and nice figures are in the UN Environmental Program report Policy Implications of Warming Permafrost