Fiat,oil level not showing correctly on the dipstick

i have a 2007 fiat Palio 1.6 petrol
it was all working fine till recently i changed my engine oil ,while the amount of oil to be filled in the engine is 4 liters , while filling 3 liters the oil dipstick is reading full . and after few hundred kms run i got engine oil low pressure warning light on my dashboard .
what could be the problem ?
iam in process of changing the engine oil and filter right now and mechanics near me are clueless regarding why this is happening .

while the culprit may be oil filter but why did the engine dip stick show full even when the oil poured in was just 70 % of