Just for reference, I was working with 9 mio pop system all alone just recently and I did get over 5 % (something like 7-8 %) daily increases with 20-ish missions done. Pratically speaking zero traffic system so no opposition. IF you were getting only 4 % increase during 2 days (averaging 2 % increase per day), it's pretty clear sign that you were been worked "against" already by someone(s), imho (assuming you were doing 10 or more mission per day). And when you went on "day off", they continued. On such relatively low pop system it could be done by 1-2 players. Specially if they added some exploration & bounty dump on top of mission running.