Definitely agree that it would be worth stopping the cereal. If he was a great sleeper, and the only thing that has changed is the solids introduction, then I would stop for a week or so and see if that makes a difference. Recommendations for solids vary from country to country (and even doctor to doctor

) but the WHO recommends exclusive breastfeeding whenever possible until 6mo of age. Some babies are ready for solids sooner (and there is a list of the signs here
http://babywhispererforums.com/index.php?topic=36922.0) but in general it's best to wait if you can.
Cereals in general are also difficult to digest (not sure what type of cereal Farex is?). Rice cereal is recommended as a first food because of its low potential for allergic reactions, but it is very tough on the digestive system, causing gas and constipation in many babies. My ds got horrible, horrible gas from it when he was younger. Other cereals can be tough too. If your lo is showing all the signs of readiness for solids you could always try offering pears. Tracy recommended them as a first food because they are mild flavoured, easy to digest, and also have a low potential for allergic reaction.
Could you post your EASY? Just in case anything jumps out that might impact your nights. Keep in mind too that there is a big growth spurt at 4mo (not sure your ds' exact age?) which might be changing your feeding routine. Teething could be a factor too, like pp suggested, have you tried meds or orajel?