My 4 month old baby wants to nurse constantly, but fusses and squirms at the over and over when I try?

It sounds to me like your baby may be getting or cutting teeth He whats to be coforted by nursing but it also hurts too. Babies can cut teeth for months before they actually brake thru. so try rubbing teething med on gums when he gets fussy like this then offer him your you will see a big difference. Worked great for me and I was going thru the same symptoms.