I'd also say cam. If you take the rocker cover off you should be able to find whichever lifters it is that are causing the noise by playing with the rockers when each respective valve is closed, if the rocker is loose with the valve shut then this means that particular lifter(s) has worn and can no longer adjust to close the gap up. Obviously you will have to spin the engine over by hand to get all the valves at the closed position.

You can also tell a lot by looking at it, take all the lifters out and spin it over and look for wear on the lobes. If the lifters are tight coming out DONT wrench them out, this means theyve worn so bad the edges have got a burr on them and you will score the lifter bores, so lift them up to clear the cam, pull the cam out and then drop the lifters in and get them out the cam shaft hole.

Of course the most accurate way is with the cam out, and use a vernier caliper to measure each lobe.

If you score the lifter bores you will make the seal between the head and the lifters poor and will increase the chance of tapping and lifter bore wear.

You will probably find its the last lobe on the cam next to the fuel pump lobe, poorly designed, the lobes on number 4 don't sit correctly underneath the lifters.