Zorg u should make teriforming more usefull make it so u get more then just 4 more feilds . It is sorta usless rate now you have to use alot of resorses just get little amount of feilds.

while 4 fields would provide 1 of each mine and a fusion reactor.
the only reason this would mater is if your a miner and for your main planet.
chances are you should have planed out your game play style before you started to colonize new planets, and should have kept deleting the small ones until you received big ones. unless if your on speed where the biggest is 300. even at that you should still have enough space for big mines and adequate amount of silo's for daily growth for up to 2 days max. anything more is risky to have even with a sick amount of defenses. death stars are cheep to send and only takes a few hours so if your leaving planets for days on end don't expect not to be hit a few times.
and if you have already hit this point in your mines you should start investing into energy tech. having it to lvl 20 plus at this stag is a big bonous for fusion reactor and should be able to handle one level of each. you my also have to deconstruct some solar plants to provide more space due to the change in power supply.
an increase in fields would be great although it should be calculated based on a percentage of growth from the planets size.
on speed an increase at 2% per level would only be 6 fields per.
with that being said on standard and extreme there are some planets that are 500 plus witch would give 10 fields. so anything more would be a little to drastic. or over powering. there is a point in this game that mine building does reach a point that its not worth building anymore. where it would take a good year of production to pay for its self. you just have to ask your self how long you really want to play this game for