* Bouteflika defends candidacy; health questions linger
* Opposition parties already seeking April 17 boycott
* Many Algerians wary of unrest after 1990s civil war (Adds more details)
By Patrick Markey
ALGIERS, March 22 (Reuters) - President Abdelaziz Bouteflika said in a letter addressed to Algerians on Saturday his poor health would not prevent him from running for a fourth term and promised constitutional reforms if he wins the April 17 election.
Bouteflika, 77, registered his candidacy earlier this month despite suffering from a stroke last year that opponents say has left him unfit to campaign or govern the North African oil producer for another five years.
In the letter published by APS state news agency, Bouteflika gave his most detailed remarks yet about his intentions although he has spoken and appeared rarely in public since the illness that put him into a Paris hospital for months.
"It would cost me dearly to remain deaf to your calls. I decided not to disappoint you and offer myself as a candidate for the presidential election ... and give all my energy to fulfilling your wishes," the letter said.
With the backing of the dominant National Liberation Front(FLN) party, loyal army factions and unions, Bouteflika is almost assured victory in Algeria, a key partner in Washington's campaign on Islamist militancy in the Maghreb.
Official campaigning starts Sunday and loyalists portray Bouteflika as the man who helped stabilise Algeria after its 1990s war with Islamist militants, a conflict that left many Algerians still wary of political upheaval.
But critics say since its 1962 independence from France, Algerian politics have been dominated by FLN elites and army generals who, while competing behind the scenes for influence, see themselves as guarantors of stability.
Since his illness, Bouteflika's allies have pushed to strengthen his position by reducing the influence of the country's powerful military intelligence chief, who for years played the role of kingmaker in Algerian politics.
Still, analysts say, those political rivalries may resurface if Bouteflika's health fades during a fourth term, though they are unlikely to threaten Algeria's stability.
Six opposition parties, including Islamist and secular movements, have announced that they will boycott April's vote which they say is unfairly tilted in FLN's favour. They remain weak and divided, and no opposition candidate is likely to seriously challenge Bouteflika.
In his letter, Bouteflika said if re-elected he would seek reforms to create a political model with different segments of society that would "meet the expectations and hopes of the people", without giving further details.
"That would be realised through a constitutional reform, which could take place during the current year," he said.
One source close to the presidency said reforms may include the creation of a vice presidential post and limit the number of times future presidents can run for office.
Algeria's election comes at a difficult time for its neighbours. Libya, Egypt and Tunisia are struggling with the instability that followed their "Arab Spring" revolts that ousted long-ruling autocrats in 2011.
With huge foreign reserves from its energy sales - around $200 billion - Algeria has spent heavily on housing and social programs to ward off protests. But social tensions over jobs, services and housing are widespread.
Bouteflika's rare public appearances have also left questions about what happens next if his health worsens, he cannot continue and has to step down after winning the election in April.
"I propose to devote this new mandate you have demanded of me to preserving our country from internal and external hostilities," the president said in his letter. (Additional reporting by Lamine Chikhi; Editing by Eric Walsh)