its hard to say but it would be an epic battle
Itachi - He has the Sharingon and the Mangekyo Sharingon and hes got Tsukuyomi which we don't know if the fourth is a genjutsu user or if he could even get out of it, Than we got the Ameterasu, and Suson'o. Itachi is one of the fastest ninja in Naruto when kakashi couldnt see his hand signs even with the sharingon, Itachi pretty much beat Diedara without any effert whats so ever (sorry diedara fans, i am one to), he was also able to defeat Orochimaru pretty easily and Orochimaru killed the Third Hokage and it is arguable to who is stronger the third or fourth.
The Fourth - Konoha's Yellow Flash, the problem is we don't know to much about the fourth, he has the flying thunder god technique, he created the rasengon and dead demon consuming seal and he can summon frogs. in the Kakashi gaiden arc, the enemy ninja saw him and stated that sight of the yellow flash they should retreat.
if they were in a fight the fourth could beat the ameterasu with the flying thunder god technique pretty easily but we dont know if he has a resistance to genjutsu and if he doesnt that would be a problem but since he was a hokage we can assume he had some. without the thunder god technique i would say itachi is faster. If they were both in their prime it would probably be the most intence battle of the series but both are dead so we wont get that dream. i think itachi would win just because i dont know to much about the fourth but if i knew more i would think otherwyse (mabye)