TL;DR: I don't have a TG-4, but I've had the Panasonic DMC-TS3, (and because of a fluke), the Olympus TG-1, TG-2, and TG-3. The Olympus TG series is awesome because it gives you freedom to shoot wherever you are, and it has the toughness and battery life to back it up. The TG-1 and TG-2 cost $399 when new, but in Olympus's attempt to lower the TG-3's price to $349, the rear screen actually got worse and overall build quality/materials suffered. The TG-3 still offers best-in-class image quality compared to other waterproof/rugged point and shoots, but it's certainly not going to out-shoot a DSLR inside a waterproof case. Low light sensitivity, microscopic macro, and effective image stabilization are more than adequate for most uses, making this a great every-day-carry adventure camera.
Some things to consider regarding these cameras:
1) Battery charging is done in-body using the included (and proprietary USB cable) and opening a waterproof door. That's actually how I got a TG-2 ~ My TG-1's electronics got fried by crappy local power in Thailand and Olympus sent me a new TG-2 as a replacement. Olympus has an official external battery charging accessory, but it's really expensive ~ I've used cheaper ones like Wasabi power and they seem to work fine. When you have this camera, do yourself a favor and get a couple sets of batteries. If you don't use GPS for geotagging and turn the camera off when you're not using it, a single battery can last hundreds of shots over a couple of days.
2) The rear screen on the TG-3 is really easy to scratch. It's plastic and it's not very tough at all. Also, while a universal problem, the screen will be next to impossible to see underwater unless you're looking directly at it.
3) The Aperture Priority mode is simulated. There's only 3 steps; f/2.0, f/2.8, and f/8.0 wide, and f/4.9, f/6.8, and f/18 zoomed, and it doesn't actually affect depth of field. More or less, it applies ND filters, meaning it maintains the same exposure but affects ISO (higher) and shutter speed (longer).
4) With these cameras, even when powered on, the exterior lens doesn't move. This helps with super-fast power-on-to-first-shot times, making the camera ready to shoot nearly instantaneously. This benefit comes with a trade-off: the majority of the lenses (focusing, zooming) and the image sensor are actually perpendicular to the image plane and all of the light goes through a prism. This optical formula degrades image quality but it also helps making the camera compact and rugged. If you're comparing just by price, a $199 point and shoot will have better image quality. Buy this camera only if you're going to really use it.
5) Olympus recommends replacing the waterproof packing and seals annually.These are wear items and you will get charged for the service.
Some things to consider regarding these cameras: 1) Battery charging is done in-body using the included (and proprietary USB cable) and opening a waterproof door. That's actually how I got a TG-2 ~ My TG-1's electronics got fried by crappy local power in Thailand and Olympus sent me a new TG-2 as a replacement. Olympus has an official external battery charging accessory, but it's really expensive ~ I've used cheaper ones like Wasabi power and they seem to work fine. When you have this camera, do yourself a favor and get a couple sets of batteries. If you don't use GPS for geotagging and turn the camera off when you're not using it, a single battery can last hundreds of shots over a couple of days. 2) The rear screen on the TG-3 is really easy to scratch. It's plastic and it's not very tough at all. Also, while a universal problem, the screen will be next to impossible to see underwater unless you're looking directly at it. 3) The Aperture Priority mode is simulated. There's only 3 steps; f/2.0, f/2.8, and f/8.0 wide, and f/4.9, f/6.8, and f/18 zoomed, and it doesn't actually affect depth of field. More or less, it applies ND filters, meaning it maintains the same exposure but affects ISO (higher) and shutter speed (longer). 4) With these cameras, even when powered on, the exterior lens doesn't move. This helps with super-fast power-on-to-first-shot times, making the camera ready to shoot nearly instantaneously. This benefit comes with a trade-off: the majority of the lenses (focusing, zooming) and the image sensor are actually perpendicular to the image plane and all of the light goes through a prism. This optical formula degrades image quality but it also helps making the camera compact and rugged. If you're comparing just by price, a $199 point and shoot will have better image quality. Buy this camera only if you're going to really use it. 5) Olympus recommends replacing the waterproof packing and seals annually.These are wear items and you will get charged for the service.