Hi Ron at 4" dia you've got quite a reduction in volume you won't be able to fit the same number of flue tubes which will reduce heat transfer to the water, you also have to be careful that you've got the correct thickness if your not to compromise strength. The boilers in simplex is a well tried and tested design I wouldn't deviate from it too much, or you may be disappointed in the performance which would be a shame after all the hard work that would have gone into it.
If you can't get tube of the correct diameter you can always roll a tube up from sheet join it together with a lap joint lots of the members at my club build great boilers that way.
Hope this helps
Stew