The preset is only a minor aspect of the video quality, at least for presets slower than "faster". The determining factor is the available bandwidth, or better the available bandwidth-per-pixel.
According to your log, you use a bandwidth of 3850 for a 1920x1080@30 video. This is only slightly above the minimum recommendations according to Twitch, which recommend 3500-5000 at 25-30 fps.
You are also using the slow preset, which puts a huge load on your processor, but if you switch to the "faster" preset, you would probably not see any difference in quality. Both presets differ in only about 1.5% better bandwidth usage but double the CPU usage.
On the other hand, if you simply use a bitrate of 5000, with faster, you would see a big difference. Unfortunately, your internet connection isn't 5mbit, only 4, so this isn't a valid option.
Your valid option is to reduce the resolution by keeping the bandwidth of 3850 (or better 3500 to get a bigger margin for network spikes). Rescale your output in Settings->Video to 1280x720 and stream this resolution. This way, you get more than double the bandwidth per pixel as with 1920x1080, thus way better visual quality. And don't use the slow preset, faster is sufficient. You gain more power available to capturing and running your apps.
