The expelled Lok Sabha members are Captain Jainarain Nishad (from Muzaffarpur), Purnmasi Ram (Gopalganj), Sushil Kumar Singh (Aurangabad) and Mangani Lal Mandal (Jhanjharpur). Three of them -- Nishad, Ram and Mandal -- were suspended earlier.
The expulsion letter issued by JD (U) general secretary K C Tyagi did not mention the reasons.
Sources said the party had information that while Nishad and Singh are likely to join BJP, Ram and Mandal are in touch with Lalu Prasad's RJD. Ram was earlier also in touch with Bharatiya Janata party (BJP), according to sources.
Shivanand Tiwari, who has off late been critical of the functioning of Bihar Chief Minister Nitish Kumar, was denied a Rajya Sabha re-nomination this time widening the gap between the two.
Suspending Nishad and Ram for six years on charge of engaging in anti-party activities, JD-U had said in October last year that action was taken against them because "both were hobnobbing with communal and corrupt forces".
Nishad had last year organised a (Narendra) 'Modi yagna', supporting the BJP's prime ministerial candidate's bid for power. Ram had also met a close aide of Modi then. Both the leaders were making statements against Nitish Kumar.
While Nishad hails from the OBC community of fishermen, Ram is a Dalit leader and BJP has been courting them as it seeks to expand its vote base in Bihar ahead of Lok Sabha elections which it will fight on its own after its split with JD(U).
Nishad was in Lalu Prasad's RJD and BJP earlier before joining JD(U) while Ram was once a minister in Prasad's cabinet in Bihar. Mandal had also been in RJD in past before joining the JD (U). Singh is a Rajput leader from Aurangbad and had been MP twice.
Mandal and Singh had also faced proceedings under anti-defection law along with then rebel MP Rajiv Ranjan Singh Lalan. The party had withdrawn the proceedings in July last year. Lalan later mended relations with the Bihar Chief Minister.
Lalan, Sushil Singh and Mangni Lal Mandal were suspended in 2012 for anti-party activities.