Ladobe - Agreed - but as time goes by the Mach IV is going to die away, now that Remington has made a legitimate round out of it. They have done the same to the 22 Varminter (22-250) both renamed but the same. 25-06 and 7-08 are two examples of wildcats that were tamed by Remington that have the same name.
Hornady 7th edition lists IMR 4198, H 4198 and RL7 as a few powders that work well with 20 grain bullets. In general the faster rifle type powders work best. Hornady is the only manual I have that lists Mach IV loads - because they are the only ones that make 17 caliber bullets that I own (Nosler, Sierra, Lyman 48th and Speer). In the Hornady manual, IMR 4198 and H 4198 are the top choices. IMR 4198 starting load at 13.8 for 3600 fps and a max of 16.2 for 4100 fps. With H 4198 Starting at 14.3 for 3600 fps and a max of 16.7 for 4100 fps. The 221 Fireball with 50 grain bullets are similar in powder weights to the 17 Fireball with 20 grain bullets - BUT NOT THE SAME. For example the 221 Fireball has a max of IMR 4198 as 16.6 grains with the 50 grain V Max. Remember, powder varies from one lot to the next and guns vary from one to the next, so listings are just a guide line. You may find you need more powder to get the same velocity or you may find that you need less to get the same velocity, so always start at the low end and work your way in small increments until you get the accuracy you want or you reach top velocities. There really is not that much difference in performance between 3900 fps and 4100 fps.
BTW - those 20 grain 17 caliber bullets are small - you might make your self a pair of small tongs to pickup bullets with and place them onto the case mouth for loading - just a thought.
Good Luck and Good Shooting