Owning your own home

Sorry as going to come back to you yesterday...

Opening them when it's hotter outside isn't recommended as I understand it.

Sort of. Firstly, the temps you mention (eg 26 at night) sounds hotter than outside, generally. Secondly - yes sure if you are actively cooling the inside, then don't open the windows. Comfort isn't just temps though, it's a combination of temp, humidity and air movement. So increasing air flow can make it feel more comfortable even if it doesn't change temp.

I don't have expertise on active cooling machines. Nor do I know much about reflective films, sorry. Taping paper on windows works, but you can't see out of them.

If I was to suggest low energy approach - or what you might try while waiting for the active cooling thing to arrive:
Day-time:

1. Reduce as much solar gain as possible. Daylight is fine, it's sunlight you want to block out.
   
2. Ventilation - In the morning, hopefully the inside is relatively cool, so as it hots up outside you might want to not let too much hot air in. But, once inside is hot too, try to encourage air-flow. One-sided ventilation only really works in a small room. It's much more effective to encourage cross-ventilation. So, you open both windows in your living room. Or open a window there, prop your doors open, open in furthest right bedroom and kitchen too.
   Your considerations there are going to be - where is the prevailing wind coming from? Where is the sun coming from? Ideally you want the incoming air to be coming from a shaded part of the building - this will change throughout the day, but it makes a difference. Note that air takes the easiest route, so you can get dead spots. Fans can help with that. South and West-facing windows are the most problematic.
   
3. Convection/ stack effect - hot air will naturally try and get out at the top of the room. So try to open windows as high as you can for air to get out (no idea what your windows are like, but trad sash are great for this). This pulls air in from any openings/vents lower down (not good for toddlers) but could also be a consideration when trying to encourage cooler/shaded air in from one place and hot air out from another place. If you only have low down openings, the hot air in the room just stays there, stuck.
   
4. Night-time - open everything up as much as you can and get through flow, try and get rid of that hot air and replace it with cooler. Assuming outside is cooler. At 4th floor you have the benefit of not so many moths/mosquitos/thieves coming in at night, and tend to have some air movement. But not much shade. Do you have any exposed thermal mass - like a concrete floor finish? Unlikely, but if so, cool that down at night as much as you can.