Lukaku failed to convert two good chances that he had into a goal, as the Inter goalkeeper deprived him from finding the back of the net. Roma tried very well in this match, because there was a time that Roma was pressuring to equalize the third goal, but all the effort that they put in went in vain.
The fourth goal was what I never expected, but it came like a shock for the host, as they lost hope on sharing points with Inter. This season Inter has scored a good number goals in all their matches, because they do score large number of goals in a match. De Rossi have witnessed his first defeat since he came to Roma, and I wasn't expecting him to win Inzaghi men. Inter keeps on going higher in points, staying on top to make sure that they end the league as the champions this season. It will be a difficult task for Juventus to bring them down from the first position on the table
The game was pretty close for a while, but this game was another example to see what it takes to beat Inter Milan. As you said Lukaku had two more chances and if you want to beat Inter Milan, those chances can't be missed or otherwise Inter Milan will immediately punish those foregone opportunities. This would have been an impressive win since I am sure that hardly anyone would have expected Roma to have a chance. But the game ended as expected eventually. Inter Milan is a different calibre and they are in great shape. Their stats are only getting better with every week passing by and they are not showing any weaknesses except for their elimination from the Coppa Italia.