My below reply is in assumption that the crank but not turn over is merely due to a low battery and the car will run if provided a new battery.
At 98k your engine is still considered new by many here. So long as it has had proper oil changes and not overheated or run out of coolant should be an easy fix. If you are not losing coolant, burning oil, or seeing excess steam/smoke come out the tail pipe it is probably a more common maintenance item-related issue and not something terminal inside the engine.
Note: cylinders are 1-5, 1 at the passenger side near the timing cover, 5 nearing the driver side.
Since p0302 and p0304 are for misfires in cylinder #2 and #4 you can mark the coils with a sharpie, move them to another cylinder, and see if the misfire follows or remains at #2 and #4. That way you know if it is a coil problem, even though you just replaced them, or something else before dropping money on parts. (Like swapping #2 with #1 and #4 with #5. “Something else” also doesn’t mean “worse”, there are simply a handful of potential causes.)
It sounds like you know your way around doing basic maintenance but here is a video for changing the coils in case.
Overall, sounds like run of the mill maintenance issue that can be fixed, just gotta figure out what.
A few other very easy things to do/look out for while you are in there to avoid future issues/costs.
1. When you remove the coils give the point where the coils bolt to and connect to the engine a good cleaning/scrub with a brush in case corrosion has built up. Poor electrical connection may cause issue.
2. Similarly, give the grounding straps/points on the left and right suspension tower a good cleaning by unbolting, probably not even all the way, scrub beneath with a metal brush, and reattach. There is also one at the back left side of the engine with a braided metal strap – make sure A. that strap is intact and B. you clean the mounting point on the head itself.
3. Give your coolant tank and coolant hoses, especially the plastic Tee fitting a good look. They are prone to breaking/leaking as these cars age and loosing coolant will = dead engine. See figure 6 in this link for the location. (just below the MAF. It may be a pain if you have the stock airbox but it is worth getting a view.
Radiator hoses can leak when they get old and dry-rotted. Here’s how to change them.
www.pelicanparts.com
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4. Give your timing belt a good look for any cracks or signs of wear/age. Volvo recommends every 6, but no more than 10, years OR 108k miles. Many here do it a little early just as a precaution. Unless the belt is visibly damaged or in poor condition I wouldn’t say it is a “OMG stop what you are doing sell the cat and get it done!” but just be aware and put it on your radar for the next year or so if it hasn’t previously been changed. Same goes for the accessory belts which are very easy to also replace on your own if they are in poor condition. Similar to loosing coolant, a broken timing belt = dead engine.
You can see how to jump the car and also see the grounding points on the strut tower in figure two of the link below. If it has a wire going to it = ground point. No wire going to it then it is the suspension, don’t undo those lol. ignore the green arrow below.
Dead battery? Need to charge or jump your C30? Look here.
www.pelicanparts.com