Leaving issues such as crosswind limits etc. aside: KOI and LSI have 1.428m and 1.500m runways. LCY has 1.508m and has seen ER3 ops by Jetmagic and Luxair. So just looking at the runway length, it should be doable for sn ER3 .For the ERJ145EP, takeoff field length TOW for 400nm, full PAX, ISA, SL is 1.410m, for MTOW it is 2.000m+. So it really depends what you are looking for, how much wear and tear you are willing to accept etc. etc., but generally it does not look as if an ER4 could be operated from LSI/KOI in an useful way.