How can I use a "cook for 3-4 hours" slow cooker recipe when I'm away for 8+ hours?
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.