R help - Oct 2011 - Equidistant time spacing of time series data

I am running ARMA model on time series data. Data consists of price changes
in S&P 500 during the day, recorded at 4 predefined time stamps( 8:30,
10:30,10:15, 3:12, hence non equidistant) 

My question is, do I need to worry about the data being non equidistant? 
If so, how do I modify my series to account for it? 
Or is it safe to assume data as being equidistant,as it may not have a
strong influence on the final forecast? 

Thanks! 

-Ritz

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