Fair Strata Split?

I live in Sydney NSW with a block of 4 apartments with 2 garages: ie two apartments have garages, and two do not. Otherwise, the apartments are the same size (it’s a classic 1930s block of four + 2 garages.)

Currently the strata fees are split 25-25-25-25 (presume it’s in the by-laws).

When I moved in 5 years ago, my solicitor says the levy spilt should be 30-30-20-20, with the garage owners paying a higher proportion of the levy. My apartment does not have a a garage, so he said I should be paying 20%, not 25%.

Naively, I was so excited about buying my first place I didn’t pay much attention, but now realise I have been paying too much (cumulatively about $4000+ over what I would have on 20%). 

We are a self-managed strata. I have just raised this issue with the three other owners and we will discuss at our next meeting. I am asking on this forum because I would like to go to the meeting informed. 

Questions:

1) What is the fair split? 
2) If my solicitor is right, how can I change the by-laws?
3) Would the new split also apply to special levies?
4) If the by-laws are changed, is there any hope of recouping the overage money I’ve already paid out? (ie, the difference between paying 25% and 20% over the years). I guess not, but wanted to double check?

Thank you so much for your help!