An A3 printer, or multi-function copier is designed to handle A3, A4, letter, legal, ledger and some smaller sizes like postcards & envelopes. An A4 laser printer, or multi-function copier, is smaller and more compact. It is designed to handle only letter & legal, but not A3 or ledger sized paper. An A4 machine will also typically handle some smaller sizes like postcards and envelopes. Which one is right for your office depends mainly on the paper sizes you will be printing, copying and scanning. However there are a few other things you should be aware of before you make a decision.
Many offices use only letter and legal sized paper. However some offices use the larger ledger size for engineering documents, making booklets, menus, and design layouts. Ledger size paper is the same size as two letter sized pages put together. This makes booklet making easy by simply folding the paper in half. Also because of the large size it is useful for enlarging engineering diagrams to a readable size. Ledger is also great for printing large spreadsheets on. Here is an example of ledger size verses 2 letter sizes.