Will the S1 accept a drop shape mast?

What year, current? North Dropshape I assume? If so I severely doubt it, the pocket is fairly tight even on an RDM on all the ones i've had, 2012-2016. I think I have the bottom half of a dropshape in the shed, i'll have a look tomorrow.

What about Hot Sails QU4D?

Not sure it would accept it or be very happy about it! I agree with CJW that the bottom section would also be too tight and could lead to damaging your nice sail! Severne masts are stiff top and the correct mast aides the "coiled spring" concept the S1's are famous for - reacting super fast to launch you at the lip!