Happiness,
I believe that you are correct, that if you don't empty your breast and leave the hind milk in the breast, then your baby will be getting insufficient calories from the breast milk and will be fussy.
You could continue, as you have before, to use your breast pump in order to empty your breast, but pump less frequently.
The principles of breastfeeding follow the laws of demand and supply, so your breast will produce the milk when your baby or pump, demand it. Thereby reducing the supply if you space out the breast expression. By only emptying half your breast, you may also become engorged or put yourself at risk of mastitis.
In summary, you have two options. start to pump and fully empty only one breast per feed / expression or pump less often, spacing out the intervals between pumping.
What supplements are you intending to give your baby? If you pump less, you will have less breast milk.
Is your baby four months old? If so, it is ideal to follow the UNICEF / World Health Organisation guidelines recommending that you exclusively breastfeed for at least until your baby is six months of age. Therefore, if you can continue to fully give your breastmilk and avoid supplementing your baby with formula or solid foods, for another two months, your baby should have had an truly excellent start to his or her life. Maybe you should delay trying to reduce your expressed breast milk until then.
