35mm slide projectors - using dissolve/programming units

Hello there,

I need some help and advice please! I am wanting to set up a 4x projector dissolve on a loop. I am using 4x elmo 252 projectors, I dont have a dissolve unit or programmer yet as I dontk now exactly which I need and where to get it.

I want my 4x 80 slide carousels all to go 1 by 1 cycling through all 4 projectors in a row, or all at once, it doesnt matter really. at an interval of say, 1 second, less would be ideal but I realise that most of the faders/potentiometers on the ones ive used so far go to 1sec as lowest.

So, my main question is, what programmer should I use for 4 ELMO projectors? Audio could also be a nice edition but it is not needed. It is for a slow animation for an art show. the projectors will be set up 2x2 on top of each other to make one large square like this:

__________
|_1__|_2__|
|_3__|_4__|
I hope you understand what Im trying to do.
Any help appreciated!

Thanks,
Emil