Lol, how sure are we that Arsenal will finish second? Arsenal second position is under threat from Newcastle United and Manchester City. It's just five points and six points that separate the second, third and fourth team respectively while we still have four more games to round up the season. The fact that we all agreed Manchester City is extremely bad this season but they're still in the top four contesting for champions league slot tells how superior they are. Look at their Manchester united counterpart having a bad season and comfortably switching positions in the third quarter table.
5 points more than a third position and 7 points more than a sixth position, Arsenal won't surely finish this season even in top five. They must play carefully because 4 matches are long enough for big surprise and Arsenal might get big shock if by whatever reason they lose 2 in 4 remaining matches. If they have terrible performances like this, just my assumption, they can have risk to fall to a fifth position and if in two other matches they won't win, they can fall to sixth position.
Arsenal have good advantage against other clubs in top five but they must secure it rather than playing carelessly and only focusing on Champions League. They have only some remaining matches and must perform as best as possible. If they lose in Champions League and have a terrible ending in Premier League like with a sixth position, it will turn this season from a successful one to a nightmare.
Personally, I believe that Arsenal have already secured their spot on the table and no club below them can take it from them. Newcastle is on the third position and only them can be a threat to Arsenal if they lose more than one more match. However, the second position has been Arsenal's birth rite for a couple of seasons now.