Why are glycosidic bonds always 1->4 in polysaccharides?
Why are glycosidic bonds always 1->4 in polysaccharides?


The premise of the question is false.


  Why not 1->3 or 1->2 ?


1,3-linked glycosidic bonds certainly exist.  In fact there's a whole class of molecules called beta glucans of which the 1,3-linked variety forms a major part.  


  Could it be that, the formation of glucose chains in plants and animals always make use of enzymes and that it is these enzymes which, by the shape of their active site, dictates what the bonds will be?


I think what you say here is true, but you should keep in mind that there are many different enzymes, and some will specifically form one linkage, and others will form alternate linkages.