Step-down fixes, as they're called, are mandatory on a non-precision approach. They're put there because of an obstacle in the final approach segment, which is why you don't see them on all approaches, only those where a specific obstacle is an issue. In the UK they always used to be underlined, ie NOT BELOW, but of late the AIS charts are produced to a different standard and you might well see them not underlined. There's also potential for confusion since the AIP sometimes now shows only the absolute minimum altitude (shown as a grey shaded rectangle under the approach segment in question) and not the altitude at the step, in which case you're supposed to follow the advisory altitudes shown (ie at 4d you should be at 1200 etc) and you're not allowed below the altitude shown for the shaded box.
Note also that there's nothing wrong with being higher than the step, and by that I mean more than 100ft above it; the important thing is that you're not below it; there are still some examiners out there who think that you have to be within 100ft of a step. Also, the concept of 'dive and drive', ie descending to the step altitude immediately after passing the previous fix, is legal but not recommended. In actual fact there's a 15% slope under fixes to consider but that's the realm of procedure design and since no aircraft is likely to descend at more than a 15% angle it's only important for theorists and procedure designers.
The hold is no longer flown as a 4-minute hold, although it normally works out like that. ICAO (PANS-OPS 8168) says only that the outbound leg must be 1 still air minute, ie adjusted for wind. The CAA examine a hold such that the outbound leg is correctly adjusted and that the inbound track is established for 'a reasonable time', normally a minimum of about 15 seconds. You shouldn't fail a hold if your total time is not 4 minutes.
If you want more info on either of these please PM me.