Even holding Warp Four for 12 hours doesn't accomplish too much. The engines might require a lot of maintenance after that, or a lot of cool down time before trying again. Meaning you could be either doing spurts of Warp Four for half a day followed by being stopped for repairs, or forced to slow down to Warp Two or Three while the engines got retuned for the next Warp Four burst of twelve hours. if she can make Warp Three between her spurts of Warp Four, the equivalent if around Warp 3.5, but probably hard on the engine. With Warp Two between spurts, she's down to Warp 3.3 equivalency. With her having to stop between spurts, she effectively can make Warp 3.2, which is about half the speed of Warp 4, and thus take twice as long to reach its destination as if she made Warp Four the whole way....and four times the time as Enterprise maintaining Warp Five the whole way, which by her third year of operation, Enterprise could sustain Warp Five for almost two months without stopping, possibly longer, save her destination was reached. By her fourth year in service, she was refit to allow her to make Warp 5.2. And while that doesn't seem like much, if one follows the Warp Factor cubed system, that's just over 140 times the speed of light rather than 125 times the speed of light. And even if one doesn't follow the cubed scale, but go for a rough equivalency, that's still a reasonable improvement (if using non-speed of plot numbers, that is 6 weeks to Vulcans at Warp 5.2 rather than 7 weeks to Vulcan at Warp 5).