Think very hard, can you find another in that shape for $4000 that will not need work? I have both 88 Formula's with $6000 each which is enough right now to buy a good replacement even though I have only $5000 total in both. $10,000 would guarantee you could buy nearly any Fiero you want in great shape and would not be seen as an excessive number as it fits in their value range. Hagerty has a $6000 minimum anyways on newer cars. Hagerty is a Broker/agency not an insurance company. All their polices say Essentia Insurance Company on them. Fiero's are creeping up in value and it is getting harder to find "good ones". Each Fiero costs me around $130/year at $6000 each coverage, I do get a multi-car discount as I have 4 collector cars on the policy. Pretty sure bumping it up to $10K each would add not much more than $50/year to the policy.
A friend had his very, very nice LS swapped, lowered and customized C10 stolen recently, it took 4 months to fight on a regular policy with comprehensive coverage to get what he said was the amount he "would have taken for it on a day he was angry with it". They offered $8k he said no way and had to get an appraisal based on photos he had of it from his instagram account. He eventually got $28k but the truck was easily worth $35-40k. He now has Grundy agreed value and an ignition disabling system in his replacement c10.