Do you want to total the data each day? Or do you want to average it? Is the data at regular 6 hour intervals, or is there a timestamp for each data item that needs to be checked because the 4 per day is not perfectly regular?
If it is completely regular, then
T = [YourData(:); nan(4,1)]; %pad data in case it is not multiple of 4 long
trimoff = mod(length(T)-1,4)+1;
T(end-trimoff+1:end) = []; %throw away padding to get to multiple of 4
data4 = reshape(T, 4, []);
data24sum = nansum(data4);
data24mean = nanmean(data4);
You can get nansum and nanmean from http://www.mathworks.com/matlabcentral/fileexchange/6837-nan-suite
If you are absolutely sure that your data is a multiple of 4 long, then
data4 = reshape(YourData,4,1);
data24sum = sum(data4);
data24mean = mean(data4);
