Can someone help me with the interpretation of this agarose electrophoresis. I don't understand what can cause the pattern of lanes 3 and 4 that I got in this gel
The concentration of the template DNA used in this PCR reaction is very high. In a normal PCR reaction, 25 to 30ng concentration is sufficient. However, in this PCR reaction, the concentration of DNA will be more than 100ng. You should use fresh buffer to run. Results are non-conclusive and fresh new electrophoresis is required here.
If your DNA ladder goes up to 10 kb then those bands higher than the ladder are probably not linear PCR product but rather your nicked plasmid DNA. Even if the molecular weight of your plasmid is like 10 kilobases, nicked plasmid DNA runs much higher.
The smiling bands in positions 3 and 4 look like there is too much salt in the sample. Try diluting the sample in water and running it shorter and slower ( less time lower voltage) to see if a proper shaped band is seen