Me and my friends play Civilization a lot and the same question appeard soon after we bought expansion.
After reading rulebook carefully, we decided, that: "In order to invest a coin, the player discards one of their coin token from play (lowering their number of coins by one) and then gains one investment marker." means, that someone just changes his or her number of coins on dial, still keeping tokens in order to know how many times someone used specific tech ability. The good thing (?) about this solution, is that no one is able to receive too many coin tokens from one tech card, what may give him or her great advantage (for example; receiving coin from Code of Laws, investing in Military-Industrial Complex, taking battle to receive another coin, and so on). But on the other hand, we have problems with counting which coins have been already invested and which not.
After reading this topic, I would agree, that we were wrong. Limit on tech cards was created becouse of economic victory, and without ability to gain more than four coins from one tech, it is hard to win the game this way. Other players will invest received coins, but someone who wants to accumulate them, will not afford him/herself to spend even one. When we assume, that there can not be more than four tokens on one tech card, but players can still receive new when they have spent the old ones, those of them who want to win by economic victory are able to invest and later accumulate fifteen coins.
Like Piotrek, sorry for my english