The balloting in the four states, involving 8.3 million voters and some 1,200 candidates, is seen as a test of the nation's pulse ahead of a string of polling in major states later in the year.
The elections in Himachal Pradesh, Nagaland, Meghalaya and Tripura are the first since the ruling BJP swept the December electoral contest in Gujarat on a wave of Hindu consolidation.
Although analysts do not see these elections as politically vital, they are crucial to judge the appeal of Prime Minister Atal Bihari Vajpayee's BJP, which has been recharged by its spectacular victory in Gujarat.
"We will win the poll, as the Congress is in a state of confusion and suffering from factionalism," BJP spokesman Mukhtar Abbas Naqvi said.
He said the party was sure to retain Himachal Pradesh despite the aggressive campaign by the Congress. "People know the kind of development work we have done in the state."
Congress spokesperson S Jaipal Reddy rejected the BJP's claims. "Gujarat will not work in other states. We are confident of winning everywhere."
The term of the 60-member Meghalaya assembly ends on March 8, that of the 68-member Himachal Pradesh assembly on March 11 and those of Nagaland and Tripura, with 60 seats each, on March 22.
The Congress party of Sonia Gandhi rules the northeastern states of Meghalaya and Nagaland while the Communist Party of India-Marxist (CPI-M) governs Tripura.
The millions of votes that will be cast Wednesday would be counted on March 1.
By-elections will simultaneously take place in six assembly constituencies spread across five states - Uttar Pradesh (Gauriganj and Haidergarh), Jammu and Kashmir (Pampore), Tamil Nadu (Sattankulam), Karnataka (Humnabad), Maharashtra (Bhokarden).
The Election Commission has put off the by-election in the Poreyahat assembly constituency in Jharkhand to March 24 due to a strike by policemen and a section of Home Guards.
Polling in three snow-bound constituencies of Himachal Pradesh, Lahaul Spiti, Kinnaur and Pangi Bharmaur, had earlier been postponed to June.
The by-polls to watch out for would be Gauriganj, which falls in the Amethi parliamentary constituency of Sonia Gandhi, and Sattankulam, where a rejuvenated Congress has taken on Tamil Nadu's ruling AIADMK party with the tacit backing of the main opposition DMK party.
The Congress has tried to counter the BJP's Hindu-tinged appeal in Himachal Pradesh with an aggressive campaign against Chief Minister Prem Kumar Dhumal's government, focussing on its alleged acts of corruption.
Both Vajpayee and Gandhi campaigned spiritedly in the hill state and, given the history of elections here, analysts are expecting a close finish. A few thousand votes here or there have the potential to make or break as voters pick from 360-odd candidates.
Though top national leaders stayed away from the northeast, the race for the ballots has been no less eventful.
In Meghalaya, the Nationalist Congress Party (NCP) - which has a ruling alliance with the Congress -- denied Chief Minister F.A. Khonglam a ticket, accusing him of corruption. But the NCP and Congress are going it alone, and the Congress has already declared that it will not tie up with the NCP post-polls.
Nagaland Chief Minister S.C. Jamir faces an opposition united under the banner of the National Democratic Alliance this time, a contrast from the last elections in 1998 when he had an easy win due to a boycott by main opposition parties.
Last month's peace talks between the oldest Naga separatist group and New Delhi may also have a bearing on the poll in the state.
The influential Isaac-Muivah faction of the National Socialist Council of Nagaland (NSCN-IM), which held talks with central government, is dead against the chief minister and has accused him of backing its rival outfit.
But the NSCN-IM's assurance of staying away from the polls has brought little comfort to a nervous Jamir.
In insurgency-hit Tripura, the citizens will pick from 250-odd contestants under the shadow of several murderous attacks on political activists.
The banned National Liberation Front of Tripura has killed some 35 activists of the ruling CPI-M, whose main challenger will be the Congress.
Tens of thousands of security personnel would be deployed in all four states. But the deployment will be most extensive in the northeast, where thousands have died in the past five decades in ethnic and separatist conflicts.
Analyst G.V.L. Narasimha Rao said while the states involved were not really mainline, the verdict in Himachal Pradesh could be important by dint of its proximity to New Delhi.
"A landslide verdict in favour of either the BJP or Congress would mean a lot for their future," he said.