From an old eHow article....

Prune occasionally. Unlike some bushes and shrubs, Gardenias actually donÂt require pruning for health, but rarther more for shape and basic maintenance. They do not need to be pruned annually.

Prune for size and shape. The real purpose of pruning a Gardenia is to remove any deadwood, and maintain the size and shape suitable for your garden. This also prevents your Gardenia from becoming too "leggy" and losing their fullness.

Gardenias set their flower buds for the following year during the fall. Prune after the blooms have wilted and faded away, usually mid to late summer to avoid cutting away the new buds.

Use sharp pruners to avoid ragged cuts. Ragged increases the chance of infection and disease.

Trim any type of branch. Unlike some shrubs which put forth blossoms only on new wood, you can prune both green (new) and brown (old) wood on a Gardenia, because the shrubs set buds on both, ensuring blooms will appear no matter where you prune.

Don't water immediately after pruning. Give the "cut" a few hours to seal over.

If I ignore the risk of cutting away the buds, which there are few of anyway, is there any danger in pruning them now? And how much can I prune? 25%? half? To the dirt? (I wouldn't do that). If I wait until fall, they will be terribly out of control. But I'm really concerned that I don't do them damage, as they are rather keepsake plants.

They would like more sun than 3 hours to do well, by the way.

The sun issue notwithstanding... since you have 4 of them - conduct an experiment and shape one of them back now until it either looks good to you (if 25% is within your comfort zone, go for it) or the plant simply isn't leggy anymore. You're in a warm enough zone to probably do no more "damage" than sacrifice some bloom.

If that works for you, prune the others back in the same manner mid to late summer after any blooms have faded. To preserve the legacy aspect of your plants, living with legginess for a few extra months seems a small price to pay.

The literature says that gardenias can be pruned as far back as you like but - ideally when they are dormant (depending upon where you live). This allows the stems to heal over when the plants aren't actively growing. Don't cut all the leaves off. Some leaves need to be left so the plant can produce food for the root system.

Plants like gardenias (and azaleas and mountain laurel and pieris and many others) appreciate full morning sun and protection from the hot afternoon sun - that is their idea of partial shade - they want the shade in the afternoon.

My gardenias got a bad case of whitefly and sooty mold several years ago and in frustration I cut them back as far as I could with the tools I had at the time, from 5' tall to about 18". (It was a good thing I had limits -- I was pretty frustrated.) They grew back beautiful and healthy and two years later you couldn't tell they'd ever been pruned. I think it was late spring when they got whacked.