Hi,
In that specific case I think the best will be specific custom field with the custom price system (like the "donation" page).
Using a plugin, you can create your own custom field and have your specific display.
Then you can display your four radios and the input ; your plugin custom field will read the radio value and set the corresponding price.
It will also check that if the fourth value is selected, the requested price is correct (not too cheap).
To create a custom field using a plugin, it requires some PHP/Development skills.
In HikaShop there is an example of custom field plugin thanks to the "advanced date picker".
For more details about HikaShop development :
www.hikashop.com/support/support/documen...r-documentation.html
Donation page example :
demo.hikashop.com/index.php/fr/hikashop/...t-page/donation-page
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.