A Colt mk 4 is not illegal in Cook County.
It may be illegal in Chicago. But there are 2 exceptions.
1. a Chicago person (Law-abiding and possessing an Illinois FOID card legitimately) Can own a pistol And have in within Chicago boundaries if it was registered with the Chicago police before 1982. ( and have the registration renewed every year there after)
2. A Foid carrying Chicagoan can own a pistol but may not keep it within Chicago's boundaries in it was not registered in 82 with the city and re registered every year there after. But They Can purchase a handgun and MUST Store it outside the Chicago city limits.
2(A). if somehow they forgot to renew the registration one year... Chicago wrote the law to NOT allow re-registration of said firearm. AKA you lose.
3*********Federal law requires out of state transfers / sales must go thru an FFl in the firearm receivers state aka (your buddy needs to get an FFL in his state to receive the firearm)
Big Cliff Note**** Illinois residents MUST possess a valid and un-expired FOID card in order to own , possess,or purchase any Firearms, ammunition, reloading supplies such as gun powder. Heck even have to have it for BB guns. Also check back of FOID card It has the states requirement for all firearms transfers. You can also find the information on the Illinois State Police web site. If seller does not have FOID card... Blade at a 45 degree angle and get the F' out of Dodge big time....
Even though Cook County and Chicago are very similar, they are not one and the same. There are other city's, townships, and villages in Cook County.
And we're not even covering a check on the serial numbers that the pistol is stolen or a crime gun or not.