Three of the worst speeders were caught on the A5 in north Wales, while the other was on the M4 in south Wales, the Institute of Advanced Motorists said.
It said the 136mph driver broke the 60mph limit in Conwy by 76mph and was one of the worst cases in Britain.
The IAM had asked the Welsh forces for their worst offenders caught on camera.
The figures for 2014, released following a freedom of information request, showed Wales' worst speeder was recorded by North Wales Police at 136mph on the A5 Ty Nant to Dinmael road in Conwy.
The A5 also saw two further instances of over 120mph speeding recorded; 122mph on the Ty Nant-Cerrigydrudion stretch in Conwy; and 121mph at Halton, Wrexham.
The final case of 120mph being exceeded was captured by South Wales Police: 125mph on the M4 between Junction 35 Pencoed and Junction 34 Miskin, where a 70mph limit is in force.
The charity is campaigning for an increase in visible policing to deter excessive speeders.
North Wales Police
South Wales Police
Gwent Police
Dyfed-Powys Police
Four people were caught driving at speeds of over 120mph on roads in Wales last year - with one topping 136mph, a road safety charity has revealed.