Quick answer
Divide the price per pound by 0.45359237 to find the price per kilogram. At $4.00 per pound, the equivalent price is about $8.82 per kilogram. Compare both offers in the same currency and weight unit.
Formula and assumptions
One pound is exactly 0.45359237 kg. Divide price/lb by this factor to get price/kg.
Worked example
5.00 per lb is approximately 11.02 per kg in the same currency.