Can someone design a schematic so I can use these 4 sirens?

 

Use a relay, wire it as follows

85 - siren output from alarm (+)
86 - Ground
87 - 12v (run from battery or othe high current wire, use a fuse)
30 - Out to all positive siren wires

Then ground each siren where ever you mount them

 

I found a schematic on 12volt

85 - alarm siren output
86 - ground
30 - fused 12v
87 - output to sirens

Do I need SPST or SPDT?

And which one is correct, or are they both?

Am I able to run 3 off the relay without trouble?

 

That way will work too. The reason I did it the other way is because then when the sirens are not going off the middle pin on SPDT relay will be not used and there for is exposed while resting at 12v the way you have it. Either relay will work I would use the Single Pole Double Throw. 3 will be fine.