We get two on Roadsters Dave although they now reside on my Land Rover giving a very grown up sound. Have one of these dual tone airhorns now on Morton.
![[Linked Image]](https://i.ebayimg.com/images/g/LXgAAOSwWSlje1RH/s-l1600.png)
Very compact single unit with integral compressor. Sad to say it gets used rather a lot as yet another driver fails to see the car.
I had a hobby refurbishing vintage horns both interrupter and awoogah types. Small selection below.
![[Linked Image]](https://i.imgur.com/g2wsJgo.jpg)
I still have a few left including the deep sounding original Lucas Alto, similar to one on the right.
ETA: note the wiring for second horn on your +4 will already be there. Look for light green/white (live +ve) and purple/black (switched ground).