I have around 32kb up/download even though I set the maximum to 4000kb up/down.
I am assuming you meant 32 kB/s and 4000 kB/s since Tixati uses bytes for that not bits.
What OS are you using?
On my win 7 I can't replicate this behavior. After adding some torrents to test, turning off the DL cap allowed 6000+ KiB/s. Setting it to 4000 KiB/s worked as expected, without killing it as shown on your graph.
One difference in settings I noticed is that you didn't have Trading reciprocity set though I can't imagine how that could be related.
I turned that off in mine for testing, and it had no effect on the DL BW. I could not produce the total drop you showed.
Please note also that when I let DL run unlimited, my UPload BW was impaired.