4 is a little young to go to disneyland... (I read an article recently that disney suggests 7 as the perfect age for kids to enjoy it) the walking around and waiting in line will make for a tired crabby kid and it'll be tough to enjoy yourselves...
Stroller would work... (for when the kid wants to walk -- I'd probably use one of those leashes for kids - yeah they look awful but it keeps your kid from wandering too far from you...)
If you google stroller rental and disney - you'll get some helpful links...
http://www.mouseplanet.com/akrock/akrock2.htm
(this link points out that the max weight in the stroller is 40 lbs...
Do you have a stoller from home you could bring-- it can either be gate checkd at the plane or even go directly into baggage...
Does the hotel provide any kid services -- like a day camp or babysitting (even for a fee) where you could leave junior for the day ad you and your husband go off and do what you want to do - like go on the rides that the 4 year old wouldn't be allowed on.