Alternating LEDs through n-MOSFETs
When V2 goes high it turns M2 on.Then drain of M2 becomes ground and 9v actually divide across R2 and R1,so gate of M4 is now 4.5 volt which is greater than threshold voltage.so M4 turns on and D1 glows.To solve try a voltage divider like R1=220,R2=22k.