GML How to make gamemaker randomly select a sprite over time

[footlegchicken]

lets say if I had: spr_chicken1, spr_chicken2 and spr_chicken3 and I want them to change randomly after 4 seconds, how do I do that?
like spr_chicken1 turns into spr_chicken2 and maybe back into spr_chicken1 then spr_chicken3 just make it go randomly

 

[obscene]

count+=1;
if count==240 { sprite_index=choose(sprites,names,here); count=0; }

 

[TheouAegis]

Or if you don't to make sure the sprite changes every time (obscene's suggestion has the potential for a sprite to remain the same for more than 4 second):

count+=1;
if count == 4*room_speed {    //this is how you check for frame-based seconds if room_speed might not be 60
    do var n = choose(sprites, names, here)
    until sprite_index != n;
    sprite_index = n;
}