Three explainations
- Misclick... meant to push 4!S but hit wrong button
- A mistaken view of how to bid (I will bid 4!H and if partner doesn't like he can correct to bid 4!S)
- UI being shared between NS ("info that 5!H and 3!S with south somehow shared")
Any of these can be the "right reason". Number one is by far and away the most likely. Nuber 2 if North is total beginner and confused, and number 3, well, let's just hope not number 3.
If I was a beating man, I would say either of first two is right answer, as with no kibitzer, and they are sharing info, south would simply bid 3H over 3D and north raise to 4H. That was priobably the auction at every table where 3D was opened (or almost every table)....
IF you are thinking this hand might play in hearts or spades, you could bid 4D as north and pass partners bid, You mioght be 4-3 heart fit rather thand 4-4 spade fit, but you could do that if you liked. Bidding 4H is not the way to get into either suit.. you will play 4H if that is your bid, as in this case.