profesora offers an interesting interpretation of number 4, but I still like the gerund interpretation better. I see 'heart' as the adjective, not 'beating'.
My heart beating is increasing. -- Beating is a gerund. 'beating' is the subect, not heart.
My heart beating is in a healthy range. -- Gerund again.
Can you feel the act of my heart beating? -- Still seems like a gerund to me.