I have never really done it before, but I have 40m to play around with. Any tips or things like that would be awesome

If buying something in multiples, buy one first, to see what the price is.

Is it better to buy things in bulk, or expensive single items?

It depends on profit margins, if a cheap item sells for 5 gp more than it buys for and you can buy 50000/4 hrs then you'll most likely be making more then flipping a single item. With my example for a small item doing a single flip over a period of 4 hours would make 250k, with at least 4 open ge slots to work with at the same time. With bigger items you can make a lot more but it can take a lot more time or it could have a higher risk. Say you buy a divine on the rise for 830m(just making up prices here) and in 3 days it's up to 850m, you may want to keep it for another day to hope for a bigger rise but when you log back on it has crashed to 815m. Now instead of being up 20m you're down 15m. I'm pretty sure I rambled more than i had to but i think you got the point.

The 4 hour limit resets if you sell the items.

Spread the risks. A very important thing to remember.