If you are looking at buying a new outboard for your 4m inflatable, or if you are looking for new inflatable deals, either as a package with boat and outboard together, or boat only / outboard only, you are going to want to make the right choice of inflatable boat deals. First choose your boat - airdeck / solid floor - base this decision on how long the boat is going to stay inflated at a time, if its more than 2 days then go for a heavy duty inflatable boat with aluminum floor, the best one I think is the seapro HD (heavy duty) range is well worth your consideration, its used by the UK fire brigade and many other search and rescue authorities, if you are looking for a boat that can be taken down and put up very quickly then an airdeck will be the best choice for you. But be cautious of this as when the size gets much over 3.8m the airdeck looses most of its intrinsic rigidity from the air floor, make sure you choose one with multiple strengthening slats under the air deck, or go for the 2012 seapro 380 super V with its deep v keel and super strength air floor. That way the boat will keep its shape (when inflated to the correct pressure) and it wont distort the boat even under tough weather conditions at planing speeds.
With the boat chosen its time to look at outboard engines for sale, the mercury/tohatsu/ mariner outboards 20hp is the best of the bunch in my opinion, the mercury and mariner outboards option has a twist grip gearshift which is excellent in rough conditions when you just want one hand on the tiller handle at all times leaving your other hand free to hold on for dear life! Its light weight - 50kg, and good smooth power delivery mea it the engine of choice. Most new inflatable boats of 3.5m and upwards will be rated to 20hp so this is the only one to choose at time of writing. If you are going to stick with a 15hp then the new Suzuki outboards is the lightest of the bunch at 44kg.