If two or more players are tied for the most Influence at the Institute, all tied players receive the higher point value (4 points per Influence). The rulebook states:

"The player(s) with the most Influence at the Institute gain 4 points for each of their Influence there. The player(s) with the second most Influence at the Institute gain 2 points per Influence. All other players gain 1 point per Influence."
Page 7

The use of "player(s)" indicates that all tied players receive the full value for their position, not a split or shared value.

This is further clarified in the example:

"Jamey and Megan each have 10 Influence tokens on the Institute, Biddy has 5, and Walter has 2. Jamey and Megan would each gain 40 points, Biddy would gain 10 points, and Walter would gain 2 points."
Page 7

Summary: All tied players for most Influence receive the full 4 points per Influence.
Source: page 7

Source: Core Rulebook (pp. 5, 6, 7)