The answer depends on the type of cut. If you have a tender cut of meat then there's no reason to cook it any longer than then desired doneness. If you are using a tough cut then there's lots of collagen that needs to be broken down, and that requires moisture and time. You want to cook it until all the collagen is broken down as that will make the meat tender. That may take 2 hours, it may take 6, it depends on the cut and the thickness.
I have a similar experience with my slow cooker, it goes way too fast. To get around this I've ended up putting in big chunks of meat and veggies because anything else is completely nuked after 8 hours! I've also thought about using a light timer to have it start 3 hours after I leave home.
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.
Best Answer
You should probably not reduce the cooking time by a lot, if at all. These recipes assume you are keeping the meet submerged in liquid at some constant temperature for several hours. Assuming your tenderloin is not somehow less thick than a 2 pound cut would be, nothing in the recipe changes. The core temperature of the cut won't take any less time to get up to temperature, and the lower thermal load from having less product will also not affect slow cooking.
Ideally, have some flexibility in when you expect to be done cooking and stop when you have reached the desired results, rather than after some arbitrary period of time.
Apart from that: if you are having problems with meats coming out dry, you might want to check out this question and answers. To summarize: cooking lean meat for long times is usually not ideal.