With 19 MHKs voting for her, Mrs Sharpe topped the poll, while Mr Henderson came second with 15.
Mr Greenhill and Mr Mercer, the two new members, received 14 and 13 votes respectively.
Mrs Sharpe, who has been an MLC since replacing David Anderson in 2018, and Mr Henderson, who was elevated from the Keys in 2015, were the only members defending seats with David Cretney and Tim Crookall not seeking a further term.
Mr Cretney retired from Tynwald after more than three decades of service while Mr Crookall withdrew his nomination and intends to stand in next year’s general election contesting the Peel and Glenfaba seats held by ministers Ray Harmer MHK and Geoffrey Boot MHK.
As members of LegCo, the four successful candidates will be expected to scrutinise government policy and, where required, kick back or amend Bills from the Keys.
Aside from Mr Henderson, none of the selected members has faced the public in an election but will take home a taxpayer-funded salary of at least £43,479. This could increase if the members take on a departmental role.
The full list of MLCs now stands at Mrs Sharpe, Mr Henderson, Mr Greenhill, Mr Mercer, Marlene Maska, Kate Lord-Brennan, Tanya August-Hanson and Jane Poole-Wilson.
The unsuccessful candidates and the number of votes they received are as follows: Zahed Miah - 11, Michelle Haywood - 9, Carole Lillywhite - 5, Danielle Bell - 3 and Haafizah Hoosen - 0.
Of Legislative Council’s 11 members, eight are elected by the House of Keys. The other three are the Attorney General (who doesn’t vote), the Bishop and the President of Tynwald.
The Legislative Council sits on Tuesdays from October to July, separate from the Keys, but once a month when they sit with the 24 MHKs to form the 35-member Tynwald.
To be elected as a member LegCo, a candidate must first be proposed by an MHK.