What is your opioid tolerance like? You shouldn't do the full 4mg unless you have a decent tolerance, so 1mg is a better place to start, or 2mg with a tolerance.
Next, DON'T HEAT THE PILLS! Hydromorphone dissolves just fine in room temperature water, and even colder. All the stuff left behind in the water is inactive ingredient that you don't want in you. Heating it would allow it to dissolve for a short period of time, only for it to precipitate out of the solution as a solid again inside your system. 4mg is such a small percentage of the actual pill that any of the stuff laying at the bottom of the solution is not going to be the actual hydromorphone, so don't worry if there is stuff left behind.
Syringe- Are you sure that it's 50ml, or is it 0.5ml? I know that the 31gauge usually comes in 0.5ml or 1ml, as I prefer 1ml but the size tip I get is usually only sold with the 0.5ml syringes. With the 0.5ml syringes I use like 45 units, leaving just enough room to register. With 1ml syringes I use about 70 units. It doesn't take much water to dissolve 4mg of hydromorphone, and if you do actually have a 50ml syringe and not a 0.5ml one then only draw up like 50 units.
Filter- You should really use a micron filter when IVing anything, especially pills. If you are not going to then you should filter the solution several times through cotton (or rizla filters if you can get them since they are more effective than cotton). So after adding the water to the crushed up pill, filter it, then squirt it back out into a new sterile glass or what have you, and repeat with a new filter.
IVing is a serious decision. Please make sure you know what you are doing before committing to trying it. Look at
this post since most of the threads below the suboxone megathreads are relevant to IVing. You should at least read the
Injection Megathread.