After Six thinks Four has been killed, he finds Four's scarf in some wreckage and wears it for awhile. Also a Tear Jerker.
When Five shows up as Ten is getting stressed over all his incarnations appearing, Ten pulls him into a hug and says:
Tenth Doctor: I haven't always been proud of you, but I often wished I was you again!
When you consider this was written before "Time Crash", which touched on how Ten admires Five (and David Tennant admires Peter Davison), this becomes rather touching.
Four uses his long scarf to wrap up Leela's recently de-aged children.
Also a rather funny moment: "Look ma, no Dalek!" And then he gets rushed by nearly every companion he has ever had. Oncoming Storm, meet the Oncoming Glomp!Oh, I have something in my eye...
When Rose is held hostage by a Dalek (Tor), Nine wants to kill his future Dalek-self rather than spare her life — but finally chooses to rather save her. Cue Ten smiling with tears in his eyes.
Even better, it's ONE that calls him out. To be fair though, the idea was that Ten's regeneration would eliminate the Dalek programming.
When Four is falling to Earth and believes it to be the end of his life, he finds that he can at least take solice in fact his place of death is his beloved planet Earth.