A magnetic loop antenna would fit your requirements, but (presuming you built it yourself) you would need to work out how to tune and rotate it remotely (there are commercial models that do all that, but in my opinion, they are too expensive).
You could install a full wavelength horizontal loop for 20m, that with an ATU could be made to work on 15 & 10m (no balun, just a Q-section in the feed). Then if you break the connector at the shack and just use the centre conductor of the wire you have a random length wire, that with an ATU (with Hi-impedance input, e.g. MFJ-993B) you could tune against a ground stake and probably use it on 40m.
Alternatively rather than breaking the coax braid out, if you connect it and the centre core together, and tune agaist that ground stake again, you have capacity hat loaded vertical that again with luck should work on 40m.
The problem you can have with löoft antennas is the closeness to all of your electrical equipment which could generate RFI, that an external antenna might not pick up so strong or even at all.
Hence if you can manage to get a wire outside to work as an antenna it will be better than a loft mounted one.
Are the gutters on your house metal or plastic? If metal, they have been known to work well as a transmitting antenna but be carefull that they are not close enough for someone to touch while you are transmitting, especially if you are in a shared building.
PS: Just a thought - watch out for metal backed insulation bats - if they are up there in the loft (as they often are these days) they will detune whatever antenna you put up there!