Can I replace 4" fiberglass batts with thicker ones in my attic?
I'm not an expert in this area but this is what I would do. If the ceiling joists are only 2x4s I would not use batts that are higher than the joists themselves and then run the additional batts perpendicular. This would leave a significant gap allowing hot/cold area to infiltrate from the sides. I would just put in the R13 in the empty bays and then run the R30 perpendicular to this. This way you have no air gaps. 

If you blew in you would eliminate all this and actually get better coverage and fill then you would with batt insulation. If you don't mind paying for someone to blow in, they would be done in a few hours and the blow in type I've had done isn't itchy so if you go up in the attic and need to push it around you aren't feeling like all prickly later. I have a two story so I had to go up in the attic with a small leaf rake to clear some of the eave vents. Since you are single story you may be able to take a leaf blower and blow any insulation from the vents from the outside.