It depends on the type of recipe. While paging through a slow cooker cookbook, I see relatively few recipes recommending only 3-4 hours on low heat. Of those, I think most fall into some categories:
(1) Drinks (mulled cider, etc.) -- most of these will probably not be harmed by extending the cooking time, though in some cases you might want to tone down spices a bit.
(2) Dishes using mostly fruits or vegetables that will turn to "mush" over 8 hours -- you might try starting with bigger chunks or pieces and perhaps refrigerate them (or even freeze, if it would be appropriate) before beginning. However, you might want to avoid starting with frozen or very cold ingredients when the dish involves stuff that tends to grow a lot of bacteria (e.g., raw meat).
(3) Dishes that begin with a lot of "pre-processed" ingredients (can of soup or dehydrated mix + precooked, presliced meat + canned vegetables) -- try beginning with less processed ingredients, like fresh raw vegetables, raw large hunks of meat, etc. If safe, refrigerated or frozen ingredients could again help.
(4) Desserts -- these will often be the most tricky. Some may be okay simmering for a long time, others probably not. If it's safe with the ingredients, again you may try starting with cold or frozen ingredients to slow cooking for a few hours.
Whether you could convert a specific recipe really depends on the type of dish. For things that you want to end up very tender or mushy or liquid anyway, you can probably cook it for 8 hours instead of 4 with few changes.
But in some cases the conversion may just be impossible, unless you can make use of a timer as mentioned in another response to start the slow cooker 3-4 hours before you'll come home. Even then, be sure it's safe for the food to sit at room temperature for a long time.
It sounds like you were expecting slow cooking to be like sous vide. Well, it's not. The point isn't controlled sub-boiling temperatures, it's something on the border between simmering and boiling for foods that just need a long time to cook at that approximate temperature.
Slow cooker recipes are not supposed to be very sensitive. They're expecting to be approximately boiling for most of the time, and the difference between low and high is pretty much whether the boil is marginal or a bit more substantial. In many cases, this just matters because the quantity in the pot varies, and it takes more to keep the stuff at the top hotter if it's farther from the bottom, or if it hasn't all cooked down into the liquid yet. In either case, it shouldn't be a full rolling boil; it's just boiling on the bottom, so the rest of the liquid is probably a bit below the boiling point. And yes, this is still slow cooking. It's not boiling fast enough to lose a huge amount of liquid (or worse, boil over) with the lid on.
I wouldn't really try to assign temperatures to slow cooker recipes. Like I said, they shouldn't be that sensitive. If your bean soup recipe didn't work, maybe it was a bad one. But "reliable" in the context of slow cooker recipes doesn't mean "exact times and temperatures". They're generally things that will be perfectly fine if you cook them 25% longer. Not everything in the kitchen has to be precise and formulaic; slow cookers and slow cooker recipes take advantage of that fact. And even if you do try to calibrate, you'll have trouble, since there's a temperature gradient from bottom to top. Unlike sous vide, a slow cooker is not constantly well-mixed. In equilibrium it'll be boiling at the bottom, and 10-20 degrees cooler at the top.
If you really wanted to use a sous vide controller, I imagine something like 95-98C would work for basically every slow cooker recipe, no matter whether they say high or low. Of course, mixing thoroughly enough to make your controller actually work, you may be overstirring whatever you're cooking. But the point is, things you cook in a slow cooker aren't really going to care much what the exact temperature is; it just matters that it's hot, near boiling, and not boiling so fast that it sticks on the bottom or loses a lot of liquid.
Finding reliable recipes... Well, it's like anything else. If you're looking on the internet, you have to learn to judge for yourself and look for warning signs, or stick to sites with lots of reviews. You also have to accept that sometimes you have to test for doneness and be flexible about time. This isn't really unusual; baking recipes should always have some kind of test ("until golden brown") and the actual baking times will vary. (With something like bean soup, sure, maybe the recipe was bad, maybe you didn't soak enough, maybe the beans were a little different. A stovetop recipe wouldn't have been precise either.) If all that isn't good enough for you, buy a slow cooker cookbook; tons of those have been published in recent years.
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There's a lot of variance in tofu, but there's usually a significant difference between firm and extra firm. The soft tofu will not hold up, it will probably dissolve after 6 hours. The firm tofu may possibly hold up, it really depends on how firm the brand makes their firm tofu, I suggest you try it and see.