Do yourself a favor and reverse convert to R12. The extra money for the R12 will be a small amount relative to all the other work and money you are putting into this project. The fittings are no problem, you will simply remove the R134 adapters.
You will, however, need to COMPLETELY flush the system. Because of the parts that you will replace, you will have most of the system apart and ready for flushing anyway.
Disconnect all the hoses and then remove the expansion valve. You can get a proper flush gun inexpensively at your auto a/c supply house. Simply open the cannister, pour in the flush agent. I prefer to use the flush agent that they sell specifically for this purpose, but mineral spirits will work.
Pour the flush agent in the cannister, put on the top and pressurize at the shrader valve on the cannister, then use the blow gun to force the flush agent through the evaporator, condensor and all hoses. Use compressed air TO THOROUGHLY REMOVE THE FLUSH AGENT! Do all this flushing and removal of flush agent BEFORE hooking up the hoses.
Once everything is flushed, use new o-rings everywhere and reassemble. When mounting the connection at the compressor, mount the manifold in place and make sure it is not cocked at an angle, before putting all the strain relief bolts in place for the hoses.
Use mineral oil. Ask your compressor supplier if the compressor already has oil and what kind it has. An R4 will probably have mineral oil and the correct amount BUT CHECK FOR SURE. The correct oil type and quantity are important.
Install your new filter drier LAST. Try to minimize the amount of time between installing the filter drier and evacuation, this way you will evacuate most of the moisture rather than loading up your new filter drier with it.
Make sure that the a/c technician that evacuates and charges KNOWS the situation about the system being completely opened and flushed. He needs to evacuate longer than you would under most circumstances.