Gengis Firstly you will never be more than 3hrs, let alone 4hrs from any airfield accross the atlantic. Secondly, by the shear fact that you are flying over such an area you are compelled to carry sufficient fuel for a max distance, low level, anti ice on divertion all in addition to usual fuel reserves.
As I said before, if the situation ever arises where a bullet needs to be fired, ie a hijack by terrorists - and an unlikely stray bullet goes through the window and the aircraft subsequently crashes - then the same result as not carrying a sky marshall occurs. At least this way there is a severe deterant and a level playing field in the case of a suicide hijack situation.
chiglet If "An Aeroplane" does not carry a lot of "Holding Fuel" and therefore has to divert after "10 mins [or so] hold" [due wx etc,] I presume that you have invented a means of "Self Levitation"
err no, I would continue to use my reserve fuel supply