You can but it will be a waste. You'll end up with a little nug on a stem. I'd say let it grow til at least 10 or 12 inches, at the very least (if its from seed). If its a clone and you have some killer nutes you can go straight to flowering. There's a grower I know of on another forum (so I wont post the link here, but msg me if interested) who goes straight from the cloner to flowering with zero veg time and he yields over a gram per watt. However, he is doing multiple clones... not just one.
How tall is the bottom of the light? How tall is the pot that its in? Find the difference in those numbers and that will tell you how much room you have vertically. I'd let it veg til its about half way to the light and then flip it. Plants still grow quite a bit i flowering so you want to prepare for that. Otherwise you'll have to bend over and tie down the top.