Assuming that you are only seeing a four degree ambient temperature change, I would imagine that your little yeasties are falling just short of their "sweet spot" overnight, but would not worry that there is anything to be alarmed about. Fermentation is still carrying on, but just not at the same levels overnight.
Even with an ambient air temperature difference of four degrees overnight, I doubt that you are seeing a full four degree swing in the temperature of the fermentation, as the amount of energy (or loss of energy) in the form of heat is not instantaneous. When all is said and done, I would not foresee there being enough of a swing to actually impact the final product. In the greater scheme of things - and as Graham points out, depending on the style - you will still end up with something tasty, most likely.
As you get further along into the obsession/hobby, fermentation chambers go a far way to help control temperature, depending on the requirements of your brew and the design of the chamber you pursue, you will be able to create more reproducible results while minimizing the influence from external factors (i.e. controlling temperature and light. As far as a blanket goes, I used to use towels early on, but that was more around controlling exposure to light in my glass carboys. From your comment to your original post, I am assuming you are using a bucket, which should be fine on its own out of direct sunlight.