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Yes lines 4a and  5a can be less than the taxable amount on the b line.   If any of the 1099R has the same amount for Gross and Taxable.  Then that gross amount is not included in 4a/5a.  It looks at each 1099R separately.  So you can end up with weird amounts.

Here are the 1040 instructions for line 4.  Line 5 is the same way.