Particular frequency band rejection in MATLAB using DWT

My problem is as follows: I decompose the 4th level detail coefficient of an ECG signal further up to the fourth level and I want to reject the approx. coeff. at this level, which corresponds to the powerline interference. I reject this by using upcoef() with 'd' and then specifying only the detail coefficient. Then I want to keep on using idwt() to get back at the previous level , but this is not being possible because of the length mismatch of the new reconstucted detail coeffs. and the original approx. coeffs. I would like to know if at all it is possible (in MATLAB) to reject a frequency band corresponding to the further decomposition of the detail coefficient, because when you go back up, the reconstructed coefficient lengths are different because of the inherent convolution operation.

I am posting my code below:

    [C1, L1]=wavedec(detrendedecg, 4, 'db8');
    [cd4]=detcoef(C1,L1,[4]);

    [aa1,dd1]=dwt(cd4, 'db8');
    [aa2,dd2]=dwt(dd1, 'db8');
    [aa3,dd3]=dwt(dd2, 'db8');
    [aa4,dd4]=dwt(dd3, 'db8');
    newdd3=idwt([],dd4,'db8');

    newdd2=idwt([aa3],[newdd3],'db8');
    newdd1=idwt([aa2],[newdd2],'db8');
    newcd4=idwt([aa1],[newdd1],'db8');
the original signal is detrendedecg. It is decomposed upto level 4 to get cd4 as detail coefficient. cd4 is decomposed again up to 4 levels to obtain dd4 as the detail coefficient at level 4. I want to reject the approx coefficient at this level, i.e., aa4. But this code doesn't help. It gives a matrix dimension mismatch error on the third line from thee end. What do I do now?

matlab