sorry, but YOU are the one doing over your hours, so just dont do it! of course if you cut down your hours, you cut down your holiday (although if you take it in weeks it'll still be the same amount of weeks, just less days because you work less days). Similarly, if you get paid less because you aredoing less hours and you are in a final salary pension scheme, you will of course lose out as your final salary will be less.
You are not being penalised at all. if all part itme workers got the same holiday and pay and benefits as full time workers, no-one would work full time would they?
You ahve 2 solutions
1) go back up to five days a week
2) stay at 4 days but take the hit in terms of pay, pension and holiday but only work 4 days!