The veteran actress will reprise her role as Martha Levinson, Lord Robert Crawley's (Hugh Bonneville) American mother-in-law, The AP reports.
MacLaine joined the cast as Cora Crawley (Elizabeth McGovern)'s mother in the first part of the ITV show's third series last year.
Series creator Julian Fellowes previously hinted to Digital Spy that MacLaine could make a return.
Meanwhile, opera singer Kiri Te Kanawa has also joined the cast as a house guest and will reportedly sing during the series.
Tom Cullen was previously announced to be joining the show as Lord Gillingham, an old friend of the Crawley family.
EastEnders actor Nigel Harman, Dame Harriet Walters, Joanna David and Julian Ovenden will also star in the upcoming fourth run, which airs later this year.
"The addition of these characters can only mean more delicious drama, which is what Downton Abbey is all about," said executive producer Rebecca Eaton.
Siobhan Finneran has confirmed that she will not return to Downton Abbey as scheming maid Miss O'Brien.