Fill void in suspend floor- 1957 Semi-Detached Bungalow

Hi, without knowing the reason why you are looking to infill a 4ft void below the kitchen whilst keeping a suspended floor elsewhere, and without sight or inspection of the property, my immediate advise would be "don't do it" without professional advise. At least get a Surveyor to visit and assess before doing anything otherwise you could create some serious consequences regards damp / dry / wet rot etc across the whole property Regards David

For what benefit? If it’s a draft problem then the best thing would be to install 100mm celotex between floor joists.

You would have to put ether 4in waste pipe going under your floor joist going from one room to your external vents them insult it all that will give you continue air flow from Room to room. But as above you would have to get professional advice or get someone out to inspect it properly