Hey rronald25.
1.Your 4G questions I can't answer since I'm not using 4G. However, it says 4G in the specs and I do trust the Co. I ordered it from, they were quite sencere.
2. Yes, I have 3G on both SIMs.
3. MagicSim is very thin, but yes, it adds some thickness to the SIM card. I have been in and out with the SIM card quite a few times for trying back and forth, and I have been thinking the way you are asking in your question, but so far, no problem, but yes, I can see the possibillity for it to happend. I can't feel any alarming resistanse when inserting it though. It is a little tricky to get it installed, and I will explain below.
4.No, it easily fits under the cover, you will not notice it.
When inserting the MagicSim I do the following: Push SIM1 end in till it hits the release spring for the SIM card, (till it stops, don't try to push it further without the SIM card on top), put the SIM card on top of the MagicSim and slide both in simultanusly till the SIM card locks in.
Also see my little description above about the app MultiSim.
I reacently received a data only SIM from the new European service Fogg, (heard about it?), and MagicSim does not seem to support data only SIMs, otherwise it is great for easily switching between two regular SIM cards. Easy to handle. Good luck if you buy it.