I added a jib from a topper (I think) sailboat. My jib is about 1.8sqm so it just about doubles the sail area but, and it is a big BUT, the extra sail in any kind of wind (and particularly on an upwind point of sail when the lateral pressure on the sail is at its greatest) puts much greater stresses onto the mast which then bends like a noodle meaning that you lose all sail shape and much of the drive in the sails. The answer to this is to support the mast with stays - not too difficult to do but it does add to the complexity of the rig making the sailing more tricky particularly when tacking/gybing.
The other aspect to all of this is that with almost twice the sail area and a whole lot more complexity in the rig it becomes much less manageable as the breeze increases to the point that you have to take sail down to avoid spending most of your time climbing back on board after each capsize so I don't think adding a jib is going to get you to 4 knots more easily than going out with just the mainsail in stronger winds. The jib will give you greater speed in light winds when the extra sail area and complexity is more easily handled by both boat and sailor. (Of course the other factor is the skill and risk profile of the sailor and I do not wish to set myself up on either score
FWIW my recommendations for getting more speed out of your kayak under sail are as follows:
1. get an Adventure - it is the fastest and the most sailable because of its daggerboard - certainly chalk and cheese with the Outback from a sailng POV
2. implement roller reefing - allows you to reduce sail on stronger wind days - and IMO it is with stronger winds that you will have your best chance of getting 4 knots or above.
3. stay the mast - this improves sail shape (and therefore performance) markedly and the improvement applies just as much to the mainsail as it does to a jib (stays are essential for a jib IMO as stated earlier). Stays may interfere with/prevent roller reefing however.
4. learn to hike out (i.e. to use your body weight to counterbalance the tendency of the boat to capsize) - essential when strong gusts keep hitting you as you try to squeeze that last 0.1 knot out of your GPS reading
5. forget the jib - go for speed with just the mainsail on windier days
Thus equipped and experienced you can get more than 4 knots but, in my experience, only under certain conditions and points of sail - e.g.
1. strong winds, broad reach, roller reefed mainsail, unstayed mast, balancing carefully
2. medium-strong wind, reach-broad reach, full mainsail, stayed mast (to improve shape), balancing carefully
... it is exciting and a bit touch and go but you can maintain quite high speeds (4-5 knots) in the right conditions if you are prepared to risk it.
Hope this helps