That really depends on what features you need. Are you going to need to setup an autoresponder series for each product? Are you going to need extensive shipping options or the ability to securly send digital downloads? Are you going to need a customer loyalty or affiliate program? I think which cart you need will depend on which features you'll need to support your business model.
You do not need a complex shopping cart for 4 products. I can help you with one for a reasonable cost if you like.
I use Magento. People say it is complicated and resource heavy. I don't think that is true at all. My poker webshop is running Magento, hosted by Wiredtree and it is superfast and reliable. Got a VPS with 512 mb. Did some essential tweaking to get this speed but i think that's a must for any website or webshop.
i think u should go with Wordpress shopping cart is enuff which one got PreOrder function? i mean built in, no need to install additional module etc...
yeap you can go with it after all it is now being widely used. However, some other may help you too and I recommend you to spend little more time with patience for it by reading and collecting more information and tips - you will get lots of good ideas then.
I have tried many different shopping carts and so far I feel Zen Cart is the best. It has many free addons that help improve functionality.