My advice is to use a dosing diagram. It’s a standardized table that shows the standard dilution, e.g. 1mg in 10ml (100mcg/ml); and the equivalent volume to a series of weights. You can approximate to the nearest weight. As per your example, a dose for a 4kg baby would be (0.04mg) which is equivalent to (0.4ml) of the diluted solution.
This strategy is suggested as a safety issue against the wrong dose calculation associated with preparation. You can find an example of such tables under (Anaphylactic Shock) topic in uptodate. Such tables are to be placed in the anaphylactic trays.
* Another option is to use the ready to use syringes of (0.15mg and 0.3mg) as per the weight ranges indicated:-
EpiPen Jr: IM, SubQ: Children 15 to 29 kg: 0.15 mg; if anaphylactic symptoms persist, dose may be repeated using an additional EpiPen Jr
EpiPen: IM, SubQ: Children ≥30 kg: 0.3 mg; if anaphylactic symptoms persist, dose may be repeated using an additional EpiPen
Finally, as of May,1 2016, ratio expression of epinephrine, e.g 1:1000, is prohibited by ISMP due to associated latent errors.
It’s advised to use (mg) for expression of doses.