It's the temperature. While it is unikely that you'll be capsized irrecoverably in 4" of water, you'll still be wet. If you get back into the boat and have a wet or drysuit on, you'll probably be fine, unless:
- you are wearing cotton in contact with your skin - it will not warm up and will sap your body of warmth and energy
- you are injured in some way - many lakes have rocks and in shallow water such as this, striking one's head is a real possibility
- you capsize in deeper water (a hole, soft muddy bottom) and are fully wet with the added problem of having to re-enter the kayak
- the kayak fails in some way (which can happen, though it is an admittedly rare event)
The key here is that if you are not dressed for immersion, you are at risk. You sound a tad hardier than most and can tolerate the cold more than others may, but that does not mean one's tolerance can't be exceeded.
If you want to know whether you can stand being in the water, the best option is to test it. Full immersion in whatever gear you use and then make the call as to how much risk you wish to take. I do this whenever I teach someone to kayak. I put them in the water and have them splash around a bit. If they can't take the temperature, we adjust the gear and make recommendations.
It's a wet sport and is, I feel, as much about getting wet at times as it is keeping the boat on an even keel (so to speak, since kayaking is not about keeping the keel parallel to the bottom).
The last thing I want to see is someone getting injured because they didn't take reasonable precautions (either due to a lack of knowledge or an excess of arrogance). However, if you intend to paddle in water below 50 degrees, you should be informed what to expect. Read the following:
Note that these are survival times (30 minutes max. in sub 50F water). Sea Kayaker did these tests with experienced sea kayakers and found that many of them were so cold in sub 50F water that they pretty much lost the use of hands in as little as 5 minutes. If one is so cold they cannot re-enter the kayak and paddle, the issue becomes one of whether or not the paddler can be found within such short timeframes.
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