Scratch

If you make all the different projects on the same scratch project, put them all at the start, then use when you click on a picture it runs a broadcast which, when received activates all the stuff in a certain thing,like:
when(name of the picture youre using as a link pressed
broadcast(give the message a name it doesnt matter so long as you can remember it)

Then a different script:
When I receive(the name of the message, you shouldnt have to type it in this time)
then whatever your project is. Or you could just put them all in a gallery, if you want it to work easily. Try out my project which has all the menus as I made them all change using broadcasts : http://scratch.mit.edu/projects/djm111/306630 Hope it Helps {

well another way is this.
Have your background and put your 4 pictures in ....uh why is a kid on scratch teaching a teacher on scratch anyway?
oh well with the pictures you could do this code in any of them and then if you have on broadcast to tell all the other pictures to hide (including that picture) so then when you broadcast that, a back sprite comes up and  there is a new background with new pictures
Now for the code this is what it is:

[blocks]<when green flag clicked>
<show>
<forever>
<if><< <mouse down?> <and><touching[mouse pointer   >>
<hide>
<broadcast[ (what you want it to be)
<end>
<end>[/blocks]

now for the when I recieve.. Stuff this I'm making a tutorial anyway!