Take space for a cupboard for small bedroom?

[Twoshoesnewshoes]

We have a small fourth bedroom. We need to keep a small double bed in there , and this leaves very little floor space as there is a box thing over the stairs which also intrudes.

the adjacent room is an ok size, so the DC in the small room has a wardrobe in his brothers room.
Wondering about taking space from the bigger room to allow a large cupboard in the little room.

floor plan attached.

the new layout would make a very short corridor bit into the bigger room, and leaves room for a double bed length wise.

is it a good idea? Any issues or problems anyone can think of?

We had rooms that had built in wardrobes in one long line, but half opened into one room, and half into the other. Does that make sense?
So, if you put a wardrobe all the way down the shared wall, but half for each room, would that work for both rooms?
I also think a wardrobe wouldn't be as deep as the block youve draw in - wardrobe aren't as deep as doors.

Can you not go for the IKEA day bed that is a single with storage underneath and a double when pulled out? You could also go for a double loft bed with wardrobe underneath.

@SpringOfContentment the bed in the bigger room is along the party wall so it would just be one back to back I think.
@sleepyscientist thank you, but the floor space is needed at the same time as the double bed

to minimise the building work, you might be able to knock a door opening into the wall at the yellow block you've drawn. but don't bother to build a whole new wall in bed3 and carve it up as an odd space. Just buy a wardrobe and put it in bed 3 with the opening facing your new doorway so that the boy in bed4 can reach in and get things. boy in bed3 will now have a view of the back of a wardrobe but that shouldn't matter too much - it could be painted or covered in posters.

this would be cheaper than building new walls and much easier to put back if your needs change or you sell one day.