We are looking for a fire escape ladder for our 4th floor flat that has a window underneath the opening window. I would like a recommendation of a ladder that would work for us.

You would need to have a dedicated "fire escape window" not the style of window you currently have. Also, fire escape windows are not installed above 4.5m from ground level. There must be a "protected stairwell" in your block of flats instead.

Emergency fire escape ladders (The ones that you store away) are not made for such heights, which is why you can not find one.

Also in the UK fire escape ladders are not recommended as very young and very old people can not use them.

I would however make sure you have working smoke alarms in your flat and also an easy to reach working torch, and that you test them on a regular basis.