. I am surprised you say that mains is required for a gas valve for the oven, .............
We have an LPG cooker (and rings) at home, this has a 220v controlled gas valve.
You cannot get a gas flow without the power switched on, and, there is a red 'escape button' by the door, so, if there is a problem you can hit the button on the way out and it cuts off the electricity and hence the gas supply.
Poblem is that when we have a power cut we dont have any gas either !!!
Depending on how 'old' it is gives variable performance - after about 3 months its octane rating starts to go down, this should'nt give any great problems in running but will make starting harder.
Re the question - yes you can use it in a 4-stroke engine but it will give a smokey exhaust, and depending on what the mix ratio was could lead to oiling up of the plug.
Suggest that you put in (say) 5 litres of new (non-mixed) petrol and then 1 litre of the old (mixed) petrol, this will have the effect of 'watering down' the oil and will improve the starting compared to using old petrol only